ar71xx: Buffalo WZR-HP-AG300H (600DHP) change button type

switch buttons should be declared as switch so the failsafe shell is not
triggered by accident in procd

Signed-off-by: Luka Perkov <luka@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@37131 3c298f89-4303-0410-b956-a3cf2f4a3e73