serial: 8250_early: Use port->regshift

earlycon initializes struct uart_port::regshift to the correct
value for UPIO_MEM32 already. Use the port field rather than
hard-coded value.

This enables broader support for various i/o access methods in
8250 earlycon (eg., omap8250 earlycon).

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