More work on the Mass Storage Host mode Class driver.

More work on the Mass Storage Host mode Class driver.

Added  ATTR_NON_NULL_PTR_ARG() attributes to the class drivers to improve user code reliability by disallowing explicit NULL pointers as pointer parameters in function calls where the parameter must not be NULL.

Disabled building of the Demos/Host/ClassDriver directory until Host Mode Class drivers are complete to prevent build errors in the meantime.