staging: rtl8712: Change return values in r8712_init_cmd_priv()

Change return values in r8712_init_cmd_priv from _SUCCESS/_FAIL to
0/-ENOMEM. Modify call site accordingly.

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