rent value
which is impossible when the qual value is calculated
taking into account noise, for example.

Since it is unused anyway, this patch deprecates it
in the hope that drivers will remove their sometimes
quite expensive calculations of the value.

I'm open to actual uses of the value, but the best
way of using it seems to be what the Intel drivers do
which should probably be generalised if we have noise
values from the hardware.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>