ar71xx: changed the EnGenius ESR900 mtdparts to match that of the ESR1750 since they have the same flash layout

Signed-off-by: Forest Crossman <cyrozap@gmail.com>

Backport of r42177

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42356 3c298f89-4303-0410-b956-a3cf2f4a3e73