imx6: fix typo on wandboard detection

Revision r41256 introduced a typo (wandaboard => wandboard), but later
revisions expect to match on "wandboard"

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41262 3c298f89-4303-0410-b956-a3cf2f4a3e73