rt2x00usb: Fix error return code

We know that 'retval = 0' because it has been tested a few lines above.
So, if 'devm_kmalloc' fails, 0 will be returned instead of an error code.
Return -ENOMEM instead.

Fixes: 8b4c0009 ("rt2x00usb: Use usb anchor to manage URB")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>