spi: mux: Fix mux interaction with fast path optimisations

The spi-mux driver is rather too clever and attempts to resubmit any
message that is submitted to it to the parent controller with some
adjusted callbacks.  This does not play at all nicely with the fast
path which now sets flags on the message indicating that it's being
handled through the fast path, we see async messages flagged as being on
the fast path.  Ideally the spi-mux code would duplicate the message but
that's rather invasive and a bit fragile in that it relies on the mux
knowing which fields in the message to copy.  Instead teach the core
that there are controllers which can't cope with the fast path and have
the mux flag itself as being such a controller, ensuring that messages
going via the mux don't get partially handled via the fast path.

This will reduce the performance of any spi-mux connected device since
we'll now always use the thread for both the actual controller and the
mux controller instead of just the actual controller but given that we
were always hitting the slow path anyway it's hopefully not too much of
an additional cost and it allows us to keep the fast path.

Fixes: ae7d2346

 ("spi: Don't use the message queue if possible in spi_sync")
Reported-by: Casper Andersson <casper.casan@gmail.com>
Tested-by: Casper Andersson <casper.casan@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220901120732.49245-1-broonie@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>