USB: serial: usb_wwan.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: "Bjørn Mork" <bjorn@mork.no>
CC: Rusty Russell <rusty@rustcorp.com.au>
CC: Mauro Carvalho Chehab <mchehab@redhat.com>
CC: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>