platform/chrome: cros_ec_spi: Move to real time priority for transfers

In commit 37a18622 ("platform/chrome: cros_ec_spi: Transfer
messages at high priority") we moved transfers to a high priority
workqueue.  This helped make them much more reliable.

...but, we still saw failures.

We were actually finding ourselves competing for time with dm-crypt
which also scheduled work on HIGHPRI workqueues.  While we can
consider reverting the change that made dm-crypt run its work at
HIGHPRI, the argument in commit a1b89132 ("dm crypt: use
WQ_HIGHPRI for the IO and crypt workqueues") is somewhat compelling.
It does make sense for IO to be scheduled at a priority that's higher
than the default user priority.  It also turns out that dm-crypt isn't
alone in using high priority like this.  loop_prepare_queue() does
something similar for loopback devices.

Looking in more detail, it can be seen that the high priority
workqueue isn't actually that high of a priority.  It runs at MIN_NICE
which is _fairly_ high priority but still below all real time
priority.

Should we move cros_ec_spi to real time priority to fix our problems,
or is this just escalating a priority war?  I'll argue here that
cros_ec_spi _does_ belong at real time priority.  Specifically
cros_ec_spi actually needs to run quickly for correctness.  As I
understand this is exactly what real time priority is for.

There currently doesn't appear to be any way to use the standard
workqueue APIs with a real time priority, so we'll switch over to
using using a kthread worker.  We'll match the priority that the SPI
core uses when it wants to do things on a realtime thread and just use
"MAX_RT_PRIO - 1".

This commit plus the patch ("platform/chrome: cros_ec_spi: Request the
SPI thread be realtime") are enough to get communications very close
to 100% reliable (the only known problem left is when serial console
is turned on, which isn't something that happens in shipping devices).
Specifically this test case now passes (tested on rk3288-veyron-jerry):

  dd if=/dev/zero of=/var/log/foo.txt bs=4M count=512&
  while true; do
    ectool version > /dev/null;
  done

It should be noted that "/var/log" is encrypted (and goes through
dm-crypt) and also passes through a loopback device.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>