sbc: detect when bitpool has changed

A2DP spec allow bitpool changes midstream which is why sbc configuration
has a range of values for bitpool that the encoder can use and decoder
must support.

Bitpool changes do not affect the state of encoder/decoder so they don't
need to be reinitialize when this happens, so the impact is fairly small,
what it does change is the frame length so encoders may change the
bitpool to use the link more efficiently.
diff --git a/sbc/sbc.c b/sbc/sbc.c
index a6391ae..77fcc5d 100644
--- a/sbc/sbc.c
+++ b/sbc/sbc.c
@@ -978,6 +978,9 @@
 
 		priv->frame.codesize = sbc_get_codesize(sbc);
 		priv->frame.length = framelen;
+	} else if (priv->frame.bitpool != sbc->bitpool) {
+		priv->frame.length = framelen;
+		sbc->bitpool = priv->frame.bitpool;
 	}
 
 	if (!output)
@@ -1050,6 +1053,9 @@
 
 		sbc_encoder_init(&priv->enc_state, &priv->frame);
 		priv->init = 1;
+	} else if (priv->frame.bitpool != sbc->bitpool) {
+		priv->frame.length = sbc_get_frame_length(sbc);
+		priv->frame.bitpool = sbc->bitpool;
 	}
 
 	/* input must be large enough to encode a complete frame */
@@ -1120,7 +1126,7 @@
 	struct sbc_priv *priv;
 
 	priv = sbc->priv;
-	if (priv->init)
+	if (priv->init && priv->frame.bitpool == sbc->bitpool)
 		return priv->frame.length;
 
 	subbands = sbc->subbands ? 8 : 4;