mmc: sdhci-xenon: fix annoying 1.8V regulator warning

mmc: sdhci-xenon: fix annoying 1.8V regulator warning

For some reason the Host Control2 register of the Xenon SDHCI controller
sometimes reports the bit representing 1.8V signaling as 0 when read
after it was written as 1. Subsequent read reports 1.

This causes the sdhci_start_signal_voltage_switch function to report
  1.8V regulator output did not become stable

When CONFIG_PM is enabled, the host is suspended and resumend many
times, and in each resume the switch to 1.8V is called, and so the
kernel log reports this message annoyingly often.

Do an empty read of the Host Control2 register in Xenon's
.voltage_switch method to circumvent this.

This patch fixes this particular problem on Turris MOX.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Fixes: 8d876bf472db ("mmc: sdhci-xenon: wait 5ms after set 1.8V...")
Cc: stable@vger.kernel.org # v4.16+
Link: https://lore.kernel.org/r/20200420080444.25242-1-marek.behun@nic.cz
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>