ums: move IO support code to common location

There's nothing Samsung-/board-specfic about the implementation of
ums_init(). Move the code into cmd_usb_mass_storage.c, so that it can
be shared by any user of that command.

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