s390/qeth: split out RX poll code

The main NAPI poll routine should eventually handle more types of work,
beyond just the RX ring.
Split off the RX poll logic into a separate function, and simplify the
nested while-loop.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>