MAINTAINERS: floppy: take over maintainership

I would like to maintain the floppy driver. After the recent fixes,
I think I know the code pretty well. Nowadays I've got 2 physical 3.5"
readers to test all the changes.

Signed-off-by: Denis Efremov <efremov@linux.com>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>