ibmveth: Do not process frames after calling napi_reschedule

The IBM virtual ethernet driver's polling function continues
to process frames after rescheduling NAPI, resulting in a warning
if it exhausted its budget. Do not restart polling after calling
napi_reschedule. Instead let frames be processed in the following
instance.

Signed-off-by: Thomas Falcon <tlfalcon@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>