ath5k: remove double cache alignment, ath_rxbuf_alloc() already does it

ath5k is using the (csz - 1) twice as ath_rxbuf_alloc() already allocates
and moves skb->data accordingly. Remove the extra (csz -1).

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>