S2io: optimizations

Hi,
Following are the optimization-related changes made in this patch.

1. Definitions of LOW and PANIC levels of the Rx buffers have changed.
2. In wait_for_cmd_complete there is no longer a writeq but just a read and wait for strobe bit to reset.
3. In s2io_isr, the isr_lock has been done away with also the NICs interrupt are no longer disabled explicitly on entering the interrupt handler and re-enabled again before leaving it.
4. Also clearing the semaphore "tasklet_status" when exiting erroneously from s2io_isr after failing fill_rx_buffer call.
5. The set/reset Tx Csum function through ethtool was added to the ethtool_ops structure.
6. Added a Rx side error code in the rx_osm_handler function.
7. No longer stopping and waking Tx queue when link state changes in s2io_link function.
8. removed the isr_lock spinlock from the s2io_nic structure.

Thanks,
Ravi