Fix uninitialized warning in mps

Reviewed by:	pjd (mentor)
Approved by:	pjd (mentor)
MFC after:	2 weeks
- Add more defines to limit USB memory usage and number of allocations
in reduced memory systems.

- Split allocation and freeing of the configuration descriptor into a separate
function, so that the configuration descriptor can be made fixed size
to save memory allocations. This applies for both device and host mode.