null_blk: boundary check queue_mode and irqmode

When either queue_mode or irq_mode parameter is set outside its
boundaries, the driver will not complete requests. This stalls driver
initialization when partitions are probed. Fix by setting out of bound
values to the parameters default.

Signed-off-by: Matias Bjørling <m@bjorling.me>

Updated by me to have the parse+check in just one function.

Signed-off-by: Jens Axboe <axboe@fb.com>