iio: sx9310: Prefer async probe

iio: sx9310: Prefer async probe

On one board I found that:
  probe of 5-0028 returned 1 after 259547 usecs

While some of this time is attributable to the pile of i2c transfers
that we do at probe time, the lion's share (over 200 ms) is sitting
waiting in the polling loop in sx9310_init_compensation() waiting for
the hardware to indicate that it's done.

There's no reason to block probe of all other devices on our probe.
Turn on async probe.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Daniel Campello <campello@chromium.org>
Link: https://lore.kernel.org/r/20200901081920.v2.1.Id02b2f451b3eed71ddd580f4b8b44b3e33e84970@changeid
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>