hwmon: (dell-smm) Cache fan_type() calls and change fan detection

hwmon: (dell-smm) Cache fan_type() calls and change fan detection

On more Dell machines (e.g. Dell Precision M3800) fan_type() call is too
expensive (CPU is too long in SMM mode) and cause kernel to hang. This is
bug in Dell SMM or BIOS.

This patch caches type for each fan (as it should not change) and changes
the way how fan presense is detected. First it try function fan_status()
as was before commit 592a084c568a ("i8k: Add support for fan labels"). And
if that fails fallback to fan_type(). *_status() functions can fail in case
fan is not currently accessible (e.g. present on GPU which is currently
turned off).

Reported-by: Tolga Cakir <cevelnet@gmail.com>
Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=112021
Cc: stable@vger.kernel.org # v4.0+, will need backport
Tested-by: Tolga Cakir <cevelnet@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>