greybus: loopback: functionally decompose calculation of turn-around times

We have a pattern similar to this over and over again gb->elapsed_nsecs =
timeval_to_ns(&te) - timeval_to_ns(&ts); good software practice dictates we
functionally decompose this. This patch decomposes into
gb_loopback_calc_latency().

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