ACPI: thinkpad-acpi: rate-limit CMOS/EC unsynced error messages

If userspace applications mess with the CMOS NVRAM, or something causes
both the ACPI firmware and thinkpad-acpi to try to change the brightness at
the same time, it is possible to have the CMOS and EC registers for the
current brightness go out of sync.

Should that happen, thinkpad-acpi could be really obnoxious when using a
brightness_mode of 3 (both EC and CMOS).  Instead of complaining a massive
number of times, make sure to complain only once until EC and CMOS are back
in sync.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Joerg Platte <lists@naasa.net>
Signed-off-by: Len Brown <len.brown@intel.com>