lantiq_dsl.sh: properly detect vdsl_cpe_control and add missing quotes

lantiq_dsl.sh didn't work with VDSL chipsets for now, fix that by
detecting whether vdsl_cpe_control or dsl_cpe_control should be used.
Also add missing quotes around shell string comparision.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43101 3c298f89-4303-0410-b956-a3cf2f4a3e73