[mac80211] add a helper that allows loading the eeprom from a MTD partition pointed to by a phandle

Signed-off-by: John Crispin <blogic@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@36194 3c298f89-4303-0410-b956-a3cf2f4a3e73