greybus: loopback: functionally decompose gb_loopback_calc_latency

The __gb_loopback_calc_latency will be useful in later patches. Provide it
here and use as intended. Later on we just want to use the timestamp
rollover detection, so split it out now.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>