floppy,{ami|ata}flop: Convert to bdops->check_events()
Convert the floppy drivers from ->media_changed() to ->check_events().
Both floppy and ataflop buffer media changed state bit and clear them
on revalidation and will behave correctly with kernel event polling.
I can't tell how amiflop clears its event and it's possible that it
may generate spurious events when polled.