USB: serial: metro-usb: move to use dev_dbg() instead of dbg()

This properly ties the driver into the dynamic debug system and provides
the needed device identification when the messages are printed out.

It also removes a ton of checkpatch warnings as well, which is always a
nice validation that it's the correct thing to do.

Cc: Aleksey Babahin <tamerlan311@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>