luci-base: fix DSL feature detection

On boards with ADSL instead of VDSL support we need to expect an
`/sbin/dsl_cpe_control` instead of an `/sbin/vdsl_cpe_control` executable.

Ref: https://forum.openwrt.org/t/dsl-line-stats/126580
Signed-off-by: Jo-Philipp Wich <jo@mein.io>