ltq-vdsl-app: do not set the reserved bit 4 in the xTSE 8

I do not know if this causes any problems now, but we should not set
it, because it is reserved. Some more recent versions of the Lantiq DSL
API driver and Control is checking if only valid bits are set.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

SVN-Revision: 48948