AR8216: don't display MIB counters if all are empty

AR8216: don't display MIB counters if all are empty

For unused switch ports all MIB values are zero. Displaying ~40 empty
MIB counters is just confusing and makes it hard to read the output of
swconfig dev <dev> show.
Therefore, if all MIB counters for a port are zero, just display
an info that the MIB counters are empty.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48910 3c298f89-4303-0410-b956-a3cf2f4a3e73