ums: remove error-checking of MMC device size

There's no reason to believe that an MMC device will incorrectly report
its capacity. Remove error checking of this value from ums_disk_init()
to simplify it.

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