mmc: mmci_sdmmc: fix power on issue due to pwr_reg initialization

This patch fix a power-on issue, and avoid to retry the power sequence.

In power off sequence: sdmmc must set pwr_reg in "power-cycle" state
(value 0x2), to prevent the card from being supplied through the signal
lines (all the lines are driven low).

In power on sequence: when the power is stable, sdmmc must set pwr_reg
in "power-off" state (value 0x0) to drive all signal to high before to
set "power-on".

To avoid writing the same value to the power register several times, this
register is cached by the pwr_reg variable. At probe pwr_reg is initialized
to 0 by kzalloc of mmc_alloc_host.

Like pwr_reg value is 0 at probing, the power on sequence fail because
the "power-off" state is not writes (value 0x0) and the lines
remain drive to low.

This patch initializes "pwr_reg" variable with power register value.
This it done in sdmmc variant init to not disturb default mmci behavior.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Link: https://lore.kernel.org/r/20200420161831.5043-1-ludovic.barre@st.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

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