serial: altera: ensure port->regshift is honored consistently

Most register accesses in the altera driver honor port->regshift by
using altera_uart_writel(). There are a few accesses however that were
missed when the driver was converted to use port->regshift and some
others were added later in commit 4d9d7d89 ("serial: altera_uart:
add earlycon support").

Fixes: 2780ad42 ("tty: serial: altera_uart: Use port->regshift to store bus shift")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>