mmc: core: Allow mmc_start_host() synchronously detect a card

When using dm-verity with a data partition on an emmc device, dm-verity
races with the initialization of the (e)MMC/SD cards. This is because the
card detection procedure is being managed from a workqueue, rather than
synchronously from the mmc host's ->probe() routine.

To allow the card detect to be synchronous, let's simply skip using the
workqueue at the first initialization attempt from mmc_start_host().

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Link: https://lore.kernel.org/r/ZCTOMVjW+pnZVGsQ@snowbird
[Ulf: Re-wrote the commit message to clarify the change]
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>