ath9k_htc: Replace a variable initialisation by an assignment in ath9k_htc_set_channel()

Replace an explicit initialisation for one local variable at the beginning
by a conditional assignment.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Reviewed-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>