r6040: only disable RX interrupt if napi_schedule_prep is successful

When receiving the first RX interrupt before the internal call
to napi_schedule_prep is successful the RX interrupt gets disabled
and is never enabled again as the poll function never gets executed.

Signed-off-by: Michael Thalmeier <Michael.Thalmeier@sigmatek.at>
Signed-off-by: David S. Miller <davem@davemloft.net>