greybus: loopback: add gb_loopback_nsec_to_usec_latency

A helper function to convert from a nanosecond value to a latency value
expressed in mircoseconds. This will be used again in subsequent patches.

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