mmc: queue: Improve error handling during allocation of bounce buffers

Allocation of previous bounce buffer in mmc_init_queue when the current
bounce buffer allocation fails was leading to a crash later in
__blk_segment_map_sg. Error handling is improved by allocating previous
bounce buffer only if the current bounce buffer allocation succeeds.

Signed-off-by: Bhuvanesh Surachari <bhuvanesh_surachari@mentor.com>
Signed-off-by: Harish Jenny K N <harish_kandiga@mentor.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>