staging: line6: replace DEBUG_MESSAGES() with dev_dbg()

The dyndbg feature allows dev_dbg() calls to be enabled/disabled at
runtime and is therefore more convenient than static debug log messages.
Use dev_dbg() instead of the line6-specific DEBUG_MESSAGES() macro.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>