USB: spcp8x5.c: remove dbg() usage

dbg() is a usb-serial specific macro.  This patch converts
the spcp8x5.c driver to use dev_dbg() instead to tie into the
dynamic debug infrastructure.

CC: Rusty Russell <rusty@rustcorp.com.au>
CC: Alan Stern <stern@rowland.harvard.edu>
CC: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>