wireless, mwifiex: drop redundant NULL test before call to release_firmware()

Since release_firmware() does its own test for NULL it is redundant to
do so before calling it.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>