mmc: mmci: Add enough delay between writes to CMD register.

On Qcom SD Card controller POWER, CLKCTRL, DATACTRL and COMMAND registers
should be updated in MCLK domain, and writes to these registers must be
separated by three MCLK cycles. This resitriction is not applicable for
other registers. Any subsequent writes to these register will be ignored
until 3 MCLK have passed.

One usec delay between two CMD register writes is not sufficient in the
card identification phase where the CCLK is very low. This patch replaces
a static 1 usec delay to use mmci_reg_delay function which can provide
correct delay depending on the cclk frequency.

Without this patch the card is not detected.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>