mmc: core: Ensure cmd_completion is initialized

mmc_test now uses mmc_start_request() to test sending commands during
"ongoing" asynchronous transfers, i.e. tests:
	Commands during non-blocking read - use Set Block Count (CMD23)
	Commands during non-blocking write - use Set Block Count (CMD23)

mmc_start_request() was not initializing cmd_completion, but cmd_completion
is used by "ongoing" transfers, so move initialization of cmd_completion
into making mmc_start_request().

Fixes: cb39f61e ("mmc: core: Export a few functions needed for blkmq support")
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>