mmccam: Add HS200 support for eMMC

[manu]

Summary

Switch eMMC cards to HS200 when both the card and the host support it,
falling back to high speed on failure. Bus settings changes now wait for
the SIM to complete, which runs from its own taskqueue. Also fix a race
where a new request could be lost when the SIM task cleared the current
request after completing it, and fail bios queued for a partition whose
switch failed.

[bz]

Would it be too much if I asked to do the factoring out of sdda_set_host_ios() upfront?

[manu]

Split out sdda_set_host_ios