ext/alsa/gstalsa.c: cast to GstClockTime to get higher granularity

ext/alsa/gstalsa.c: cast to GstClockTime to get higher granularity

Original commit message from CVS:
* ext/alsa/gstalsa.c: (gst_alsa_samples_to_timestamp):
cast to GstClockTime to get higher granularity
* ext/alsa/gstalsasink.c: (gst_alsa_sink_check_event):
use gst_element_set_time_delay to get the exact time
* ext/mad/gstmad.c: (gst_mad_chain):
use the negotiated rate instead of the current frame's rate which
might be wrong because of bit errors. This avoids emitting totally
bogus timestamps and screwing sync.
(fixes #143454)