staging: rtl8712: return -ENOMEM instead of -1

After the commit 91d435f replaced _malloc with kmalloc, smatch picked up a
couple of new warnings. This fixes the warning:

warn: returning -1 instead of -ENOMEM is sloppy

Signed-off-by: Vitaly Osipov <vitaly.osipov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/staging/rtl8712/usb_ops_linux.c b/drivers/staging/rtl8712/usb_ops_linux.c
index 064dc10..a6c2aab 100644
--- a/drivers/staging/rtl8712/usb_ops_linux.c
+++ b/drivers/staging/rtl8712/usb_ops_linux.c
@@ -499,7 +499,7 @@
 	if (palloc_buf == NULL) {
 		dev_err(&udev->dev, "%s: Can't alloc memory for vendor request\n",
 			__func__);
-		return -1;
+		return -ENOMEM;
 	}
 	pIo_buf = palloc_buf + 16 - ((addr_t)(palloc_buf) & 0x0f);
 	if (requesttype == 0x01) {