USB: qcserial.c: remove dbg() usage

dbg() is a usb-serial specific macro.  This patch converts
the qcserial.c driver to use dev_dbg() instead to tie into the
dynamic debug infrastructure.

CC: Thomas Tuttle <ttuttle@chromium.org>
CC: Alan Stern <stern@rowland.harvard.edu>
CC: Paul Gortmaker <paul.gortmaker@windriver.com>
CC: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>