thinkpad_acpi: save kbdlight state on suspend and restore it on resume

Override default LED class suspend/resume handles, by keeping track of
the brightness level before suspending so that it can be automatically
restored on resume by calling default resume handler.

Signed-off-by: Marco Trevisan (Treviño) <mail@3v1n0.net>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>