libertas: clean up lbs_hard_start_xmit()

Having merged the nest of functions into one, now we can clean it up and
fix the error handling, and the duplication -- and at least make a start
on the locking.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>