greybus: hid: Replace WARN_ON() with dev_err()

WARN_ON() is a bit harsh here, as we just failed to power-off the HID
device while it is getting removed.

Replace it with dev_err().

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>