mmc: core: Keep host claimed in mmc_rescan() while calling host ops

As mmc_claim_host() invokes pm_runtime_get_sync() for the mmc host device,
it's important that the host is kept claimed for *all* accesses to it via
the host_ops callbacks.

In mmc_rescan(), the ->card_event() and the ->get_cd() callback are being
invoked without claiming the host, let's fix this.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>