osmo_oap_decode(): Use common argument ordering

In general, if a function generates output data like a msgb (or in this
case filling an osmo_oap_message structure), the output argument
precedes the source.  This is what we use all over libosmo*, and it is
modelled after memcpy(), where dst is the first argument, before src.

Let's align osmo_oap_decode().  Intestingly, osmo_oap_encode was already
correct, so the encode/decode functions used different conventions
before.
diff --git a/openbsc/src/gprs/oap_messages.c b/openbsc/src/gprs/oap_messages.c
index 53ea9ac..d81723f 100644
--- a/openbsc/src/gprs/oap_messages.c
+++ b/openbsc/src/gprs/oap_messages.c
@@ -31,8 +31,8 @@
 #include <stdint.h>
 
 
-int osmo_oap_decode(const uint8_t *const_data, size_t data_len,
-		    struct osmo_oap_message *oap_msg)
+int osmo_oap_decode(struct osmo_oap_message *oap_msg,
+		    const uint8_t *const_data, size_t data_len)
 {
 	int rc;
 	uint8_t tag;