serial: 8250: Simplify capabilities reset at port unregister

Since uart->port.type has just reset to PORT_UNKNOWN, capabilities
are reset to uart_config[PORT_UNKNOWN].flags, which is 0.

Removes unnecessary dependency on uart_config[].

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>