ACPI: thinkpad-acpi: clean-up fan subdriver quirk

Better document the Unitialized HFSP quirk, and modularize it a bit.
This makes the code flow easier to read and reduces LOC.

Apply the Unitialized HFSP closer to the source (i.e. inside the
get_fan_status()), this fixes a harmless buglet where at driver init
with the quirk active, the user could set the hwmon pwm1 attribute and
switch out of pwm1_mode=2 to pwm1_mode=0 without changing pwm1_mode
directly.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Tino Keitel <tino.keitel@tikei.de>
Signed-off-by: Len Brown <len.brown@intel.com>