disk: get_device_and_partition() return value fixes

When no valid partitions are found, guarantee that we return -1. This
most likely already happens, since the most recent get_partition_info()
will have returned an error. However, it's best to be explicit.

Remove an unnecessary assignment of ret=0 in the success case; this value
is over-written with the processed partition ID later.

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