FAT: check for partition 0 not 1 for whole-disk fs

The recent switch to use get_device_and_partition() from do_fat_ls()
broke the ability to access a FAT filesystem directly on a whole device;
FAT only works within a partition on a device.

This change makes e.g. "fatls mmc 0:0" work; explicitly requesting
partition ID 0 is something that get_device_and_partition() fully
supports. However, fat_register_device() expects partition ID 1 to be
used in the full-disk case; partition ID 1 was previously implicitly
specified when the user didn't actually specify a partition ID. Update
fat_register_device() to expect the correct ID.

This change does imply that if a user explicitly executes "fatls mmc 0:1"
then this will fail, and may be a change in behaviour.

Note that this still prevents "fatls mmc 0:auto" from working. The next
patch will fix that.

Signed-off-by: Stephen Warren <swarren@nvidia.com>