serial: 8250_dw: Don't use UPF_FIXED_TYPE

Allow 8250.c to determine the port type for us. This allows
the driver take advantage of FIFO on Designware UARTs that
have it.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>