wifi: wfx: Remove redundant NULL check before release_firmware() call

release_firmware() checks for NULL pointers internally so checking
before calling it is redundant.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Minghao Chi <chi.minghao@zte.com.cn>
Acked-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220606014237.290466-1-chi.minghao@zte.com.cn