staging: rtl8712: Change return values of r8712_getrfreg_cmd()

Change return values of r8712_getrfreg_cmd from _SUCCESS/_FAIL to
0/-ENOMEM respectively. Modify call site accordingly.

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