ums: remove ums_disk_init()

Now that ums_disk_init() is so simple, there's no need for it to be a
separate function. Instead, just add it to the tail end of ums_init().

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