Use more standard types for manipulating pointers.

• Use a uintptr_t cast to get the virtual address of a pointer in USB_P2U() instead of a ptrdiff_t.

• Add offsets to a char * pointer directly without roundtripping the pointer through a ptrdiff_t in USB_ADD_BYTES().

Reviewed by: imp, hselasky
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D27581