ath9k_htc: avoid kernel panic in ath9k_hw_reset

hw pointer of ath_hw is not assigned to proper value
in function ath9k_hw_reset what finally causes kernel panic.
This can be solved by proper initialization of ath_hw in
ath9k_init_priv.

Signed-off-by: Marek Puzyniak <marek.puzyniak@tieto.com>
Acked-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>