af_lavcac3enc: change default bitrate to 640

No reason to use less.

Since the name "default" is misleading now, replace it with "auto"
(still recognize the old name).

DOCS/man/af.rst

@@ -77,10 +77,12 @@ Available filters are:
     ``bitrate=<rate>``
         The bitrate use for the AC-3 stream. Set it to 384 to get 384 kbps.
 
+        The default is 640. Some receivers might not be able to handle this.
+
         Valid values: 32, 40, 48, 56, 64, 80, 96, 112, 128,
         160, 192, 224, 256, 320, 384, 448, 512, 576, 640.
 
-        The special value ``default`` selects a default bitrate based on the
+        The special value ``auto`` selects a default bitrate based on the
         input channel number:
 
         :1ch: 96

audio/filter/af_lavcac3enc.c

@@ -312,12 +312,13 @@ const struct af_info af_info_lavcac3enc = {
     .priv_size = sizeof(struct af_ac3enc_s),
     .priv_defaults = &(const struct af_ac3enc_s){
         .cfg_add_iec61937_header = 1,
+        .cfg_bit_rate = 640,
         .cfg_min_channel_num = 5,
     },
     .options = (const struct m_option[]) {
         OPT_FLAG("tospdif", cfg_add_iec61937_header, 0),
         OPT_CHOICE_OR_INT("bitrate", cfg_bit_rate, 0, 32, 640,
-                          ({"default", 0})),
+                          ({"auto", 0}, {"default", 0})),
         OPT_INTRANGE("minch", cfg_min_channel_num, 0, 2, 6),
         {0}
     },