ums: support block devices not MMC devices

The USB Mass Storage function could equally well support a SATA device
as support an MMC device. Update struct ums to contain a block device
descriptor, not an MMC device descriptor.

Cc: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Przemyslaw Marczak <p.marczak@samsung.com>