Fix -Wvoid-pointer-to-enum-cast warnings.

This pattern is used in callbacks with void * data arguments and seems
both relatively uncommon and relatively harmless. Silence the warning
by casting through uintptr_t.

This warning is on by default in Clang 11.

Reviewed by: arichardson
Obtained from: CheriBSD (partial)
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D24425