greybus: timesync: do not print frametime by default

Currently frametime are being printed whenever an Interface is
runtime-resumed. This is unnecessarily chatty. This patch moves the
frametime print from using pr_info() to dev_dbg().

Testing Done:
 - $ echo "module greybus +p" > /d/dynamic_debug/control

[   97.699395] greybus 1-svc: greybus frametime: ap=2042778707
1-svc=2042775185 greybus1=2042778397 1-6.6=2042778390

Signed-off-by: David Lin <dtwlin@google.com>
Reviewed-by: Jeffrey Carlyle <jcarlyle@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>