staging: wilc1000: wilc_msgqueue.c: remove kfree NULL check

This patch removes kfree NULL check.
WARNING: kfree(NULL) is safe and this check is probably not required.

Signed-off-by: Chaehyun Lim <chaehyun.lim@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>