The behavior of the SMC has changed several times over the years,
causing read failures in the driver. It seems the problem can be
explained by a shift in SMC speed combined with improper action on
status codes.

We should first wait for the SMC to settle, which was the most
frequent response on the old slow machines. Then, if the SMC is busy,
we need to try again later by resending the command. This was the most
likely response until 2012. Now, with a shorter wait time, we are
again most likely to poll while the SMC is settling, and as a result
we see high failure rates on many old and new models.

With the distinction between busy and failure, we can also wait longer
before retrying, without sacrificing speed.  This seems to bring
failures down to virtually zero on all models.

Tested on: MBA1,1 MBA3,1 MBA5,1 MBA5,2 MBP9,2
Tested-by: Adam Somerville <adamsomerville@gmail.com>
Tested-by: Hubert Eichner <hubert.georg.eichner@gmail.com>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>