Now that all usb-serial modules are only using dev_dbg()
the debug module parameter does not do anything at all, so
remove it to reduce any confusion if someone were to try to
use it.

CC: Matthias Bruestle and Harald Welte <support@reiner-sct.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>