ath5k: fix uninitialized var warning for txf2txs

  CC [M]  drivers/net/wireless/ath/ath5k/reset.o
drivers/net/wireless/ath/ath5k/reset.c: In function ‘ath5k_hw_init_core_clock’:
drivers/net/wireless/ath/ath5k/reset.c:100:51: warning: ‘txf2txs’ may be used uninitialized in this function

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