kern: Remove leftover saf1761otg bits

Almost all code related to the saf1761 driver was removed in commit
44796b7e822e, except for two small bits related to saf1761otg support.
This patch completes the removal.

PR: 279302
Signed-off-by: Joshua Kinard <freebsd@kumba.dev>
Reviewed by: mhorne
MFC after: 3 days
Fixes: 44796b7e822e ("mips: remove saf1761")

(cherry picked from commit 80828c6fab0292b5c5a34a63558d837cb9308fbd)