staging: bcm2835-audio: interpolate audio delay

When the BCM2835 audio output is used, userspace sees a jitter up to 10ms
in the audio position, aka "delay" -- the number of frames that must
be output before a new frame would be played.
Make this a bit nicer for userspace by interpolating the position
using the CPU clock.
The overhead is small -- an extra ktime_get() every time a GPU message
is sent -- and another call and a few calculations whenever the delay
is sought from userland.
At 48,000 frames per second, i.e. approximately 20 microseconds per
frame, it would take a clock inaccuracy of
20 microseconds in 10 milliseconds -- 2,000 parts per million --
to result in an inaccurate estimate, whereas
crystal- or resonator-based clocks typically have an
inaccuracy of 10s to 100s of parts per million.

Signed-off-by: Mike Brady <mikebrady@eircom.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>