device_to_disk: recognise /dev/ada[0-9] on kfreebsd

kFreeBSD since 9.0 deprecated /dev/ad[0-9] in favour of /dev/ada[0-9];
the former is a symlink which means device_to_disk gets called with the
latter.

Before this change, the function would wrongly return "/dev/ada" rather
than "/dev/ada0".  This broke the detection logic for $bootdev