greybus: loopback: handle timestamp roll-over

This patch ensures we account for roll-over in the loopback driver.
do_gettimeofday() is used to grab a timestamp. Two timestamps are derived
one before and one after a gb_operation_sync(), however since
do_gettimeofday() returns the number of seconds and mircoseconds that have
elapsed today - we need to account for a situation where the timestamp
starts at say 23:59:999us rolls over and the end time is now earlier than
the start time.

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