bna: remove redundant NULL test before release_firmware()

release_firmware() does its own NULL test so explicit test before call
is unneeded.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: Rasesh Mody <rmody@brocade.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>