i2o_block: Convert to bdops->check_events()

Convert from ->media_changed() to ->check_events().

i2o_block buffers media changed state and clears it after reporting.
It will behave correctly with kernel event polling.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Markus Lidel <Markus.Lidel@shadowconnect.com>