floppy: cleanup: make next_valid_format() not rely on current_drive anymore

Now the drive is passed in argument so that the function does not
use current_drive anymore.

Link: https://lore.kernel.org/r/20200331094054.24441-21-w@1wt.eu
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Denis Efremov <efremov@linux.com>