subchan_demux: Use 'ubit_t' for unpacked bit buffer; use const

Change-Id: Ia082b9fddf03d02afd007825a1588a3ef0dbedae
diff --git a/src/trau_frame.c b/src/trau_frame.c
index 717bc32..eaced3d 100644
--- a/src/trau_frame.c
+++ b/src/trau_frame.c
@@ -37,7 +37,7 @@
  *  \file trau_frame.c
  */
 
-static uint32_t get_bits(const uint8_t *bitbuf, int offset, int num)
+static uint32_t get_bits(const ubit_t *bitbuf, int offset, int num)
 {
 	int i;
 	uint32_t ret = 0;