mpc85xx: use wpad-mini in WDR4900 profile instead of wpad

This change makes the profile of WDR4900 more consistent with other router
specific profiles.

Signed-off-by: Philipp Borgers <borgers@mi.fu-berlin.de>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42896 3c298f89-4303-0410-b956-a3cf2f4a3e73