ACPI: thinkpad-acpi: fix brightness_set error paths

The code calling brightness_set() can't handle EINTR/ERESTARTSYS well, nor
is it checking brightness_set() return status properly.

Fix it.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>