Revert "gsmtap_makemsg_ex: NULL for unknown chan_type"

Reverting due to being completely normal for some applications to submit
usning this channel type when decoding/parsing failed and hence not
being able to determine channel type.

This reverts Change-Id Ib4147a33a75c3cf425c30da8b0678c7fba8a371d
(commit 12f93be6073ed89055862baafbfe0e08e50ed17a.)

Change-Id: I6ebaa89cc3422df1d7fd2365a6176cb2902d1c67
diff --git a/src/gsmtap_util.c b/src/gsmtap_util.c
index 8674f1c..9a0ac02 100644
--- a/src/gsmtap_util.c
+++ b/src/gsmtap_util.c
@@ -169,8 +169,7 @@
  *  \param[in] snr Signal/Noise Ratio (SNR)
  *  \param[in] data Pointer to data buffer
  *  \param[in] len Length of \ref data
- *  \return dynamically allocated message buffer containing data,
- *          or NULL for unknown chan_type
+ *  \return dynamically allocated message buffer containing data
  *
  * This function will allocate a new msgb and fill it with a GSMTAP
  * header containing the information
@@ -183,9 +182,6 @@
 	struct gsmtap_hdr *gh;
 	uint8_t *dst;
 
-	if (chan_type == GSMTAP_CHANNEL_UNKNOWN)
-		return NULL;
-
 	msg = msgb_alloc(sizeof(*gh) + len, "gsmtap_tx");
 	if (!msg)
 		return NULL;