Fix reverted floppy-device permissions

Fix reverted floppy-device permissions

Commit d93fb59c50a720e2a1ace2177cc824c00ff8eed6 ("rules: set mode of
floppy device nodes to 0660") changed the create_floppy_devices -M
argument to 0660, for udev-148.

Commit 78171d9549fafdfc1e29d30915104a3ad55e116b reverted this, when it
moved the call to create_floppy_devices out to another rules file, for
udev-150.

Presumably the 0660 mode was correct, and the 0640 reversion was not
intentional; this restores 0660.

Signed-off-by: Bryan Kadzban <bryan@kadzban.is-a-geek.net>