staging: brcm80211: remove NULL pointer checks before calling kfree

kfree function can handle NULL pointer as passed parameter so there
is no need for the calling function to check for this before calling
kfree.

Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Brett Rudley <brudley@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>