channel.c: Route all control frames to a channel through the same code.

Frame hooks can conceivably return a control frame in exchange for an
audio frame inside ast_write().  Those returned control frames were not
handled quite the same as if they were sent to ast_indicate().  Now it
doesn't matter if you use ast_write() to send an AST_FRAME_CONTROL to a
channel or ast_indicate().

ASTERISK-25582

Change-Id: I5775f41421aca2b510128198e9b827bf9169629b