sbc: use an uint16 to store frame length in internal frame structure

Otherwise it could overflow in some cases.
For instance in DUAL_CHANNEL mode, with subbands set to SBC_SB_8, blocks
set to SBC_BLK_16 and bitpool set to 64 results in a frame length of 268.
diff --git a/sbc/sbc.c b/sbc/sbc.c
index e830388..606f11c 100644
--- a/sbc/sbc.c
+++ b/sbc/sbc.c
@@ -119,7 +119,7 @@
 	uint8_t subbands;
 	uint8_t bitpool;
 	uint16_t codesize;
-	uint8_t length;
+	uint16_t length;
 
 	/* bit number x set means joint stereo has been used in subband x */
 	uint8_t joint;