USB: serial: cp210x.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: Johan Hovold <jhovold@gmail.com>
CC: Preston Fick <preston.fick@silabs.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>