cmd: Add dependency for ums command

Add the missing dependency for the command ums:
- CONFIG_BLK: call of blk_* functions in usb_mass_storage.c
- CONFIG_USB_GADGET: required to select CONFIG_USB_FUNCTION_MASS_STORAGE

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>