usb: usbtmc: Add vendor specific/asynchronous read/write ioctls

- ioctl USBTMC_IOCTL_WRITE sends an (a)synchronous generic message to bulk OUT. The message is split into chunks of 4k (page size). Message size is aligned to 32 bit boundaries.

With flag USBTMC_FLAG_ASYNC the ioctl is non blocking. With flag USBTMC_FLAG_APPEND additional urbs are queued and out_status/out_transfer_size is not reset. EPOLLOUT | EPOLLWRNORM is signaled when all submitted urbs are completed.

- ioctl USBTMC_IOCTL_WRITE_RESULT copies current out_transfer_size to given __u64 pointer and returns current out_status of the last USBTMC_IOCTL_WRITE call.

- ioctl USBTMC_IOCTL_READ reads an (a)synchronous generic message from bulk IN. Depending on transfer_size the function submits one (<=4kB) or more urbs (up to 16) with a size of 4k.

The flag USBTMC_FLAG_IGNORE_TRAILER can be used when the transmission size is already known. Then the function does not truncate the transfer_size to a multiple of 4 kB, but does reserve extra space to receive the final short or zero length packet. Note that the instrument is allowed to send up to wMaxPacketSize - 1 bytes at the end of a message to avoid sending a zero length packet.