nFAPI/PNF: Correctly set message length when segmenting

Make the same change as in parent commit, i.e., write the full 32-bit
segment size.

I could not test this, as we do not reach these rx_data.indication
length. It's based on the fix in the parent commit, and to avoid future
bad surprises.

nfapi/open-nFAPI/pnf/inc/pnf_p7.h

@@ -57,7 +57,7 @@ typedef struct {
 
 typedef struct {
 	uint8_t* buffer;
-	uint16_t length;
+	uint32_t length;
 } pnf_p7_rx_message_segment_t;
 
 typedef struct pnf_p7_rx_message pnf_p7_rx_message_t;