staging: rtl8712: rtl871x_mlme.c: use !x in place of NULL comparison

Challenge suggested by coccinelle.
Avoid NULL comparison, compare using boolean operator.

Signed-off-by: Payal Kshirsagar <payal.s.kshirsagar.98@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>