rtspsrc: Use the correct type for storing the max-rtcp-rtp-time-diff property

It's an integer property and rtpbin also expects an integer. Passing it
as a GstClockTime (guint64) to g_object_set() will cause problems, and
on big endian MIPS apparently causes crashes.

Fixes gstreamer/gst-plugins-good#737

Part-of: <gstreamer/gst-plugins-good!744>