media: stk1160: Restore deleted comment in stk1160_fill_urb()

This comment explains why we return success when usb_alloc_noncoherent()
fails and I think it's helpful.  It was deleted during a recent refactor,
but let's add it back.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Xu Yang <xu.yang_2@nxp.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>