floppy: use common function to check if floppies can be registered

The same checks to see if a drive can be or is registered are
repeated through the code, factor out the checks in a common function
and replace the repeated checks with it.

Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>