net: meth: Add set_rx_mode hook to fix ICMPv6 neighbor discovery

SGI IP32 (O2)'s ethernet driver (meth) lacks a set_rx_mode function, which
prevents IPv6 from working completely because any ICMPv6 neighbor
solicitation requests aren't picked up by the driver.  So the machine can
ping out and connect to other systems, but other systems will have a very
hard time connecting to the O2.

Signed-off-by: Joshua Kinard <kumba@gentoo.org>
Signed-off-by: David S. Miller <davem@davemloft.net>