ath5k: use bool type for no_hw_rfkill_switch module parameter

Avoids this:

  CC [M]  drivers/net/wireless/ath/ath5k/base.o
drivers/net/wireless/ath/ath5k/base.c: In function ‘__check_no_hw_rfkill_switch’:
drivers/net/wireless/ath/ath5k/base.c:85:1: warning: return from incompatible pointer type

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