mmc: sdhci-tegra: get rid of special PRESENT_STATE register handling

sdhci-tegra provides a get_ro method, which overrides the checking
of the write protect bit in the PRESENT_STATE register in sdhci.c:

        if (host->flags & SDHCI_DEVICE_DEAD)
                is_readonly = 0;
        else if (host->ops->get_ro)
                is_readonly = host->ops->get_ro(host);
        else
                is_readonly = !(sdhci_readl(host, SDHCI_PRESENT_STATE)
                                & SDHCI_WRITE_PROTECT);

This means it's pointless detecting accesses to this register and
manually setting the SDHCI_WRITE_PROTECT as it has no effect.

This means that the whole of tegra_sdhci_readl() can be removed and
we can use the builtin sdhci readl functionality here.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>