platform/x86: GPD pocket fan: Set speed to max on get_temp failure

When we fail to get the temperature, assume the worst and set the speed
to max.

While at it introduce a define for MAX_SPEED.

Cc: James <kernel@madingley.org>
Suggested-by: James <kernel@madingley.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>