etherh: Add MAINTAINERS entry for etherh

During the re-organization of Ethernet drivers, the MAINTAINERS
entry for etherh got dropped accidentally.

CC: Russell King <linux@arm.linux.org.uk>
CC: Joe Perches <joe@perches.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>