staging: greybus/loopback: use ktime_get() for time intervals

This driver is the only one using the deprecated timeval_to_ns()
helper. Changing it from do_gettimeofday() to ktime_get() makes
the code more efficient, more robust against concurrent
settimeofday(), more accurate and lets us get rid of that helper
in the future.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>