ath9k: fix excessive BAR sending when a frame exceeds its retry limit

Because the sendbar variable was not reset to zero, the stack would send
Block ACK requests for all subframes following the one that failed, which
could mess up the receiver side block ack window.

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