usb: ums: allows using every mmc device with ums.

Before this change ums command only allowed use of mmc 0.
Now this argument can be set.

Changes:
- remove mmc device number checking because it is always positive number
- remove printing "no such device" - it is done by find_mmc_device()

Change-Id: I767e45151ad515c7bef19e6c13087374f5e23c11
Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Marek Vasut <marex@denx.de>