ACPI: thinkpad-acpi: fix fan sleep/resume path

ACPI: thinkpad-acpi: fix fan sleep/resume path

This fixes a regression from v2.6.27, caused by commit
5814f737e1cd2cfa2893badd62189acae3e1e1fd, "ACPI: thinkpad-acpi:
attempt to preserve fan state on resume".

It is possible for fan_suspend() to fail to properly initialize
fan_control_desired_level as required by fan_resume(), resulting on
the fan always being set to level 7 on resume if the user didn't
touch the fan controller.

In order to get fan sleep/resume handling to work right:

1. Fix the fan_suspend handling of the T43 firmware quirk. If it is
still undefined, we didn't touch the fan yet and that means we have no
business doing it on resume.

2. Store the fan level on its own variable to avoid any possible
issues with hijacking fan_control_desired_level (which isn't supposed
to have anything other than 0-7 in it, anyway).

3. Change the fan_resume code to me more straightforward to understand
(although we DO optimize the boolean logic there, otherwise it looks
disgusting).

4. Add comments to help understand what the code is supposed to be
doing.

5. Change fan_set_level to be less strict about how auto and
full-speed modes are requested.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Reported-by: Tino Keitel <tino.keitel@tikei.de>