ath10k: fix wrong initialization of struct channel

chandef is initialized with NULL and on the very next line, we are using it to
get channel, which is not correct. Channel should be initialized after
obtaining chandef.

Found by cppcheck:

ath/ath10k/mac.c:839]: (error) Possible null pointer dereference: chandef

Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>