USB: serial: option.c: remove debug module parameter

Now that the dbg() macro is no longer being used in the driver,
the debug module parameter doesn't do anything at all.  So remove
it so as to not confuse people.

CC: Matthias Urlichs <smurf@smurf.noris.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>