platform/x86: dell-ddv: Fix cache invalidation on resume

If one or both sensor buffers could not be initialized, either
due to missing hardware support or due to some error during probing,
the resume handler will encounter undefined behaviour when
attempting to lock buffers then protected by an uninitialized or
destroyed mutex.
Fix this by introducing a "active" flag which is set during probe,
and only invalidate buffers which where flaged as "active".

Tested on a Dell Inspiron 3505.

Fixes: 3b7eeff9

 ("platform/x86: dell-ddv: Add hwmon support")
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20230218115318.20662-1-W_Armin@gmx.de
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>