mvsw6171: note support for 88E6172 switches

The '6171 and '6172 are similar enough to work
without any changes to the code.

Signed-off-by: Claudio Leite <leitec@staticky.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43753 3c298f89-4303-0410-b956-a3cf2f4a3e73