staging: rtlwifi: Return -ENOMEM instead of -1

Use the ENOMEM constant instead of -1 for kzalloc failure checks in
rtl_halmac.c

Signed-off-by: Aymen Qader <qader.aymen@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>