s390/qeth: unify RX-mode hashtables

To keep track of the addresses programmed from an RX modeset, we have
two separate hashtables (L2: mac_htable, L3: ip_mc_htable).

These are never used at the same time, so unify them into a single
rx_mode_addrs hashtable.

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