[POWERPC] Convert adb.c to use kthread API and not spin on ADB requests

This converts adb.c to use the kthread API.

It also changes adb_request so that if the ADBREQ_SYNC flag is
specified, we now sleep waiting for the request to finish using an
on-stack completion rather than spinning.  To implement this, we now
require that if the ADBREQ_SYNC flag is set, the `done' parameter must
be NULL.  All of the existing callers of adb_request that pass
ADBREQ_SYNC appear to be in process context and have done == NULL.
Doing this allows us to get rid of an awful hack in adb_request()
where we used to test whether the request was coming from the adb
probe task and use a completion if it was, and otherwise spin.

This also gets rid of a static request block that was used if the req
parameter to adb_request was NULL.  None of the callers do that any
more, so the static request block is no longer necessary.

Signed-off-by: Paul Mackerras <paulus@samba.org>