block/floppy: remove redundant variable dflags

Variable dflags is being assigned but is never used hence it is redundant
and can be removed.

Cleans up clang warning:
warning: variable 'dflags' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>