s390/qeth: align L2 and L3 set_rx_mode() implementations

Once all of qeth_l3_set_rx_mode()'s single-use helpers are folded back
in, the two implementations actually look quite similar. So improve the
readability by converting both set_rx_mode() routines to a common
format.

This also allows us to walk ip_mc_htable just once, instead of three
times.

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