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>

SVN-Revision: 43101