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: Johan Hovold <jhovold@gmail.com>
CC: Mark Ferrell <mferrell@uplogix.com>
CC: Donald Lee <donald@asix.com.tw>
CC: Tony Zelenoff <antonz@parallels.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>