ath9k_hw: fix smatch warning in ar9003_hw_spur_mitigate_mrc_cck

drivers/net/wireless/ath/ath9k/ar9003_phy.c:211
ar9003_hw_spur_mitigate_mrc_cck() error: potential NULL dereference
'spur_fbin_ptr'.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>