acer-wmi: Rename mail LED correctly & remove hardcoded colour

The mail LED name for acer-wmi currently hardcodes in the colour as green.
This is wrong, since many of the newer laptops now come with an orange
LED, and we have no way of telling what colour is used on a given system.

Also, rename the mail LED to be inline with the current recommendations of
the LED class documentation.

Signed-off-by: Carlos Corbacho <carlos@strangeworlds.co.uk>
Signed-off-by: Len Brown <len.brown@intel.com>