staging: rtl8712: Change return values of r8712_init_evt_priv()

Change return values of the function r8712_init_evt_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>