audioparsers: Use the peer caps for restrictions instead of the srcpad allowed caps

audioparsers: Use the peer caps for restrictions instead of the srcpad allowed caps

Otherwise we will intersect with the srcpad template caps and add all the caps fields
that the parser will ever set, no matter if downstream restricts this field or not.
This requires upstream to set this field on the caps to successfully negotiate.

https://bugzilla.gnome.org/show_bug.cgi?id=690184