aaf: Add presentation time tolerance

aaf: Add presentation time tolerance

Different AVTP applications have different presentation time tolerance.
The current version of the plugin doesn't support any tolerance so this
patch extends the AAF plugin in order to enable the user to configure
that tolerance value.

The presentation time tolerance is specified in microseconds and it is
relevant only when the plugin is operating in capture mode.  For more
information see the 'Plugin Configuration' session in doc/aaf.txt

This patch also does some code refactoring and encapsulates all
presentation time validation code in the new is_ptime_valid() helper
function.

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>