netifd: fix typo in dhcp script

git-svn-id: svn://svn.openwrt.org/openwrt@37051 3c298f89-4303-0410-b956-a3cf2f4a3e73
AA: netifd: backport r37051

git-svn-id: svn://svn.openwrt.org/openwrt@37052 3c298f89-4303-0410-b956-a3cf2f4a3e73