mmc: core: use usleep_range rather than HZ magic in mmc_delay()

Documentation/timers/timers-howto.txt recommends to use usleep_range for
delays 1-20ms. Let's adhere to it. No need for messing with HZ and still
do busy looping these days.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>