floppy: cleanup: expand macro DRS

This macro doesn't bring much value and only slightly obfuscates the
code by silently using global variable "current_drive", let's expand it.

Link: https://lore.kernel.org/r/20200224212352.8640-8-w@1wt.euSigned-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>