staging: rtl8712: Change return values of r8712_setrfreg_cmd()

Change return values of function r8712_setrfreg_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>