linux/generic: add MIB counters and port status to ADM6996 switch

linux/generic: add MIB counters and port status to ADM6996 switch

This patch adds port status information and MIB counters to the ADM6996
switch driver.

The driver supports also the older ADM6996L-variant, but I'm not able to
test this patch on that chip. According to the datasheet the same
registers exist there as well, so I think it should work, but any
feedback is appreciated.

Signed-off-by: Matti Laakso <malaakso at elisanet.fi>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40542 3c298f89-4303-0410-b956-a3cf2f4a3e73