mmc: core: Make host->card as NULL when card is removed

Make host->card as NULL when card is removed otherwise
we do see some kernel crashes in async contexts,
where host->card is referred (as dangling pointer).

Change-Id: Ifff6c7dffa674459add8b1e179f3ff7a8ddc7071
Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>