mmc: core: Introduce host claiming by context

Currently the host can be claimed by a task.  Change this so that the host
can be claimed by a context that may or may not be a task.  This provides
for the host to be claimed by a block driver queue to support blk-mq, while
maintaining compatibility with the existing use of mmc_claim_host().

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>