ath9k: remove warnings related to signed/unsigned type mismatch

CC [M]  drivers/net/wireless/ath/ath9k/recv.o
drivers/net/wireless/ath/ath9k/recv.c: In function `ath_rx_prepare':
drivers/net/wireless/ath/ath9k/recv.c:208: warning: comparison is always true due to limited range of data type
drivers/net/wireless/ath/ath9k/recv.c:220: warning: comparison is always false due to limited range of data type

Signed-off-by: John W. Linville <linville@tuxdriver.com>