6rd: Clear correct IPv6 address bits in 6rdcalc

The correct bits are now cleared in the IPv6 address as the shift
value to the correct byte in the IPv6 address was wrong. Depending
on the stack values this could result in a hanging 6rdcalc program
due to an endless loop.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38818 3c298f89-4303-0410-b956-a3cf2f4a3e73