usb_master: Remove misleading comment in do_async_bulkin()

Leftover transactions are already handled safely, so the FIXME comment
describing the issue no longer applies.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
usb_master: Rename do_async_bulkin to do_async_bulk_transfers

This function handles both IN and OUT transactions, so rename it to reflect
that and to avoid confusion.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>