blk-mq: code clean-up by adding an API to clear set->mq_map

set->mq_map is now currently cleared if something goes wrong when
establishing a queue map in blk-mq-pci.c.  It's also cleared before
updating a queue map in blk_mq_update_queue_map().

This patch provides an API to clear set->mq_map to make it clear.

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>