intrng: Use less confusing return value for intr_pic_add_handler

Currently intr_pic_add_handler either returns the PIC you gave it (which
is useless and risks causing confusion about whether it's creating
another PIC) or, on error, NULL. Instead, convert it to return an int
error code as one would expect.

Note that the only consumer of this API, arm64's gicv3_its, does not use
the return value, so no uses need updating to work with the revised API.

Reviewed by: markj, mmel
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D33341