mmc: sdhci-pci-gli: GL9767: Fix low power mode on the set clock function

mmc: sdhci-pci-gli: GL9767: Fix low power mode on the set clock function

On sdhci_gl9767_set_clock(), the vendor header space(VHS) is read-only
after calling gl9767_disable_ssc_pll() and gl9767_set_ssc_pll_205mhz().
So the low power negotiation mode cannot be enabled again.
Introduce gl9767_set_low_power_negotiation() function to fix it.

The explanation process is as below.

static void sdhci_gl9767_set_clock()
{
	...
        gl9767_vhs_write();
        ...
	value |= PCIE_GLI_9767_CFG_LOW_PWR_OFF;
        pci_write_config_dword(pdev, PCIE_GLI_9767_CFG, value); <--- (a)

        gl9767_disable_ssc_pll(); <--- (b)
        sdhci_writew(host, 0, SDHCI_CLOCK_CONTROL);

        if (clock == 0)
                return;  <-- (I)

	...
        if (clock == 200000000 && ios->timing == MMC_TIMING_UHS_SDR104) {
		...
                gl9767_set_ssc_pll_205mhz(); <--- (c)
        }
	...
	value &= ~PCIE_GLI_9767_CFG_LOW_PWR_OFF;
        pci_write_config_dword(pdev, PCIE_GLI_9767_CFG, value); <-- (II)
        gl9767_vhs_read();
}

(a) disable low power negotiation mode. When return on (I), the low power
mode is disabled.  After (b) and (c), VHS is read-only, the low power mode
cannot be enabled on (II).

Reported-by: Georg Gottleuber <ggo@tuxedocomputers.com>
Fixes: d2754355512e ("mmc: sdhci-pci-gli: Set SDR104's clock to 205MHz and enable SSC for GL9767")
Signed-off-by: Ben Chuang <ben.chuang@genesyslogic.com.tw>
Tested-by: Georg Gottleuber <ggo@tuxedocomputers.com>
Cc: stable@vger.kernel.org
Message-ID: <20241025060017.1663697-1-benchuanggli@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>