floppy: cleanup: expand macro FDCS

Macro FDCS silently uses identifier "fdc" which may be either the
global one or a local one. Let's expand the macro to make this more
obvious.

Link: https://lore.kernel.org/r/20200224212352.8640-2-w@1wt.eu


Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>