mmc: block: Fix error path of mmc_blk_alloc_req()

In case of any error within mmc_blk_alloc_req(), the bitmaps
that keep track of devices are not getting cleared. This may
result in failure to detect a card in case it reaches maximum
devices limitation. Fix it by clearing those bitmaps appropriately.

CRs-fixed: 563264
Change-Id: I0e23c45856355565534146f5fabb957fd4b1d007
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>