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

sys/dev/usb/usb_core.h

@@ -58,11 +58,11 @@
 
 /* helper for converting pointers to integers */
 #define	USB_P2U(ptr) \
-  (((const uint8_t *)(ptr)) - ((const uint8_t *)0))
+  ((uintptr_t)(ptr))
 
 /* helper for computing offsets */
 #define	USB_ADD_BYTES(ptr,size) \
-  ((void *)(USB_P2U(ptr) + (size)))
+  ((void *)(__DECONST(char *, (ptr)) + (size)))
 
 /* debug macro */
 #define	USB_ASSERT KASSERT