mmc: core: Fix error when SD card is removed

The device reboots when the SD card is removed.
This is because the host->claimer resource has been released
earlier as part of the SD card removal process.
When mmc_try_claim_host tries to check the host->claimer->task,
it dereferences a NULL pointer.

Change-Id: I8625b17e337248791de6fac2ba5b3a2d5e82894e
Signed-off-by: Bao D. Nguyen <nguyenb@codeaurora.org>