USB: ftdi_sio.c: remove dbg() tracing calls

dbg() was used a lot a long time ago to trace code flow.  Now that we have
ftrace, this isn't needed at all, so remove these calls.

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>