Document all bsg_setup_queue() arguments as required.

Fixes: aae3b069 ("bsg: pass in desired timeout handler") # v5.0.
Reviewed-by: NChaitanya Kulkarni <chiatanya.kulkarni@wdc.com>
Signed-off-by: NBart Van Assche <bvanassche@acm.org>
Signed-off-by: NJens Axboe <axboe@kernel.dk>