floppy: cleanup: expand macro UDP

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

Link: https://lore.kernel.org/r/20200224212352.8640-4-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>