odhcp6c : Silence mtu write error warnings

Silence warning "daemon.notice netifd: wan6 (1139): sh: write error: Invalid argument"
when an invalid MTU is received via RA as kernel refuses to accept IPv6 mtu values
which are smaller than 1280 and bigger than the device mtu.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

git-svn-id: svn+ssh://svn.openwrt.org/openwrt/trunk@49054 3c298f89-4303-0410-b956-a3cf2f4a3e73