staging: rtl8712: mp_start_test(): Change return values

Change return values of function mp_start_test() from _SUCCESS/_FAIL to
0/-ENOMEM and modify call sites accordingly.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>