USB: serial: ftdi_sio.c: remove dbg() usage

dbg() was a very old USB-serial-specific macro.
This patch removes it from being used in the
driver and uses dev_dbg() instead.

CC: Uwe Bonnes <bon@elektron.ikp.physik.tu-darmstadt.de>
CC: Simon Arlott <simon@fire.lp0.eu>
CC: Andrew Worsley <amworsley@gmail.com>
CC: "Michał Wróbel" <michal.wrobel@flytronic.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>