fdc: check if media rate is correct before doing any transfer

The programmed rate has to be the same as the required rate for the
floppy format ; if that's not the case, the transfer should abort.
This check can be disabled by using the 'check_media_rate' property.

Save media rate value only if media rate check is enabled.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
