ath9k: Fix uninitialized variable in ath9k_has_tx_pending()

drivers/net/wireless/ath/ath9k/main.c: In function ‘ath9k_has_tx_pending’:
drivers/net/wireless/ath/ath9k/main.c:1869: warning: ‘npend’ may be used uninitialized in this function

Introduced by commit 10e23181 ("ath9k:
optimize ath9k_flush").

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>