switch: allow Ethernet port LEDs to show specific port speeds only

switch: allow Ethernet port LEDs to show specific port speeds only

This patch adds speed_mask special file to LEDs connected to switch ports
via 'switch' trigger. It allows to choose which speeds to signal when link
is up. If router has more than one LED per port, they may light up
differently depending on how fast connection is. Default setting is 'all
speeds' so backward compatibility with system scripts (for example uci) is
maintained.

Signed-off-by: Michal Cieslakiewicz <michal.cieslakiewicz@wp.pl>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48775 3c298f89-4303-0410-b956-a3cf2f4a3e73