Quick and effective handle_tx_event() cleanup

Hi,

I mentioned that I came up with a simple way to clean up the messy (and buggy) event handling loop. Here are the patches.

Total line count is reduced by 26 and the sole loop remaining has no breaks or continues and 8 lines of code. Some functionally duplicate code is merged into one path. No functional change besides bugfixes.

Six defects identified by code review are resolved along the way. I successfully reproduced #1, and #4 was seen in the wild on linux-usb.

1. Error mid TD followed by Missed Service is misreported as missed.
2. If EP stops on the next TD after error-mid-TD, neither TD is handled.
3. Empty list quiety ignored after short TD on hosts without the quirk.
4. Emergency stall recovery not attempted after "TRB not part of TD".
5. A race could prematurely complete a TD after an isoch ring underrun.
6. Error-mid-TD transfer on buggy HC is stuck forever if it's the last.

Debugging of TD skipping is improved - we know if/how many TDs were skipped, in addition to whether a match was found or not. This enables quickly catching cases when suspiciously many TDs are skipped (I have seen a case of 150 skipped TDs, turned out to be a HW bug.)

The event handling process becomes linear - check a condition, handle something, check for another condition, handle it, and so on. This is much easier to reason about and to modify.

To demonstrate this point, patch 5/5 fixes a pair of stupid issues by inserting one check, which would be duplicated 3 times before cleanup.

This series should be a good base for future work to resolve remaining bugs. For example, the skipping loop could change from (simplified):

    while (td && !trb_in_td(td, ep_trb_dma))
to
    while (td && trb_after_td(td, ep_trb))

subject to providing a working implementation of trb_after_td(). I have tested three implementations, some based on pre-scanning the list and some on direct comparison, but I'm not 100% happy with any so far.

Mathias had a clever idea to use ring segment numbers for this. I tried and it compiled and worked flawlessly on the first go, but it requires passing all those seg pointers and total ring size around. This happens to complicate sharing implementation with trb_in_td(), because users of the latter don't currently provide such information. And I would like to share implementation of these functions, as they are very similar.

Regards,
Michal