mmc: Throttle calls to MMC_SEND_STATUS during mmc_do_erase()

This drastically reduces the rate at which the MMC_SEND_STATUS cmd polls
for completion of the MMC Erase operation.  The patch does this by adding
a backoff sleep that starts by sleeping for short intervals (128-256us),
and ramps up to sleeping for 32-64ms.

Even on very quickly completing erase operations, the loop iterates a few
times, so not too much extra latency is added to these commands.

For long running discard operarations, like a full-device secure discard,
this change drops the interrupt rates on my single-core NXP I.MX6UL from
45000/s to about 20/s, and greatly improves system responsiveness.

Signed-off-by: Martin Hicks <mort@bork.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>