Remove unused error return from API that cannot fail

No implementation of fpu_kern_enter() can fail, and it was causing needless
error checking boilerplate and confusion. Change the return code to void to
match reality.

(This trivial change took nine days to land because of the commit hook on
sys/dev/random. Please consider removing the hook or otherwise lowering the
bar -- secteam never seems to have free time to review patches.)

Reported by: Lachlan McIlroy <Lachlan.McIlroy AT isilon.com>
Reviewed by: delphij
Approved by: secteam (delphij)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D14380