libblkid: remove test for non-zero head count in FAT superblock probe.

libblkid: remove test for non-zero head count in FAT superblock probe.

This change enables the auto-detection of the internal storage on a VTech
Kidizoom camera, which has a zero head count value in the superblock.

Microsoft documentation, "FAT: General Overview of On-Disk Format", states
that the sectors and heads fields are only relevant for media that have
a geometry.

The Linux kernel also does not require a non-zero head count, and can
manually mount the filesystem.

Signed-off-by: Nick Holloway <Nick.Holloway@pyrites.org.uk>