USB: spcp8x5: pass usb-serial port to control functions

Pass usb-serial port to the control functions for uart status and work
mode.

Use port device for debugging and use dev_err to report errors.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>