powerpc/opal: Make __opal_async_{get, release}_token() static

There are no callers of both __opal_async_get_token() and
__opal_async_release_token().

This patch also removes the possibility of "emergency through
synchronous call to __opal_async_get_token()" as such it makes more
sense to initialise opal_sync_sem for the maximum number of async
tokens.

Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>