ide-floppy: do not map dataless cmds to an sg

since it fails the virt_to_page() translation check with DEBUG_VIRTUAL
enabled.

Signed-off-by: Borislav Petkov <petkovbb@gmail.com>
[bart: backport to Linus' tree]
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>