ath9k: simplify ath_tx_form_aggr

The check for ATH_AMPDU_SUBFRAME_DEFAULT is unnecessary, since it's set
to half the maximum BlockAck Window size, which is already the maximum
value that h_baw could possibly have. Also remove unnecessary variables.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>