s390/qeth: remove redundant if-clause in RX poll code

Whenever all completed RX buffers have been processed
(ie. rx->b_count == 0), we call down to the HW layer to scan for
additional buffers. If no further buffers are available, the code
breaks out of the while-loop.

So we never reach the 'process an RX buffer' step with rx->b_count == 0,
eliminate that check and one level of indentation.

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