ath5k: correct packet length in tx descriptors

Packet length calculation (which includes frame check sequence)
should take into account whether we add a pad field or not.
Extract the calculation into a helper and use it in both places.

Changes to desc.c
Changes-licensed-under: ISC

Changes to ath5k.h, base.c
Changes-licensed-under: 3-Clause-BSD

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>