ramips: Add swconfig support to ramips_esw.c

Add swconfig support to ramips_esw.c

This patch adds swconfig support for ramips_esw:

Tested on both D-LINK DIR-300 B1 and Sitecom WL-351 (external
rtl8366rb on internal port 5).
I've made sure that in the enable_vlan=0 case it behaves like a dumb
switch, so external switches should work fine with vlans and
verified this on the WL-351.

The current state shown by swconfig is always read directly from HW
registers, new settings only show after 'swconfig dev rt305x set apply'.

Signed-off-by: Tobias Diedrich <ranma+openwrt@tdiedrich.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@33299 3c298f89-4303-0410-b956-a3cf2f4a3e73