thinkpad_acpi: Don't yell on unsupported brightness interfaces

The thinkpad_acpi driver currently emits error messages on unsupported
brightness interfaces, giving the impression that someone will implement
those. However, this error is spit out on nearly every thinkpad in
production since 2 years now. Furthermore, the backlight interfaces on
those devices are supported by the i915 driver just fine.

Downgrade the error message to a normal pr_info() and stop telling people
to report it to IBM.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>