6lowpan: udp use subtraction on both conditions

Cleanup code to handle both calculation in the same way.
Signed-off-by: NAlexander Aring <alex.aring@gmail.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

net/ieee802154/6lowpan_iphc.c

@@ -557,7 +557,7 @@ static void compress_udp_header(u8 **hc06_ptr, struct sk_buff *skb)
 		lowpan_push_hc_data(hc06_ptr, &tmp, sizeof(tmp));
 		tmp = /* subtraction is faster */
 		   (u8)((ntohs(uh->dest) - LOWPAN_NHC_UDP_4BIT_PORT) +
-		       ((ntohs(uh->source) & LOWPAN_NHC_UDP_4BIT_PORT) << 4));
+		       ((ntohs(uh->source) - LOWPAN_NHC_UDP_4BIT_PORT) << 4));
 		lowpan_push_hc_data(hc06_ptr, &tmp, sizeof(tmp));
 	} else if ((ntohs(uh->dest) & LOWPAN_NHC_UDP_8BIT_MASK) ==
 			LOWPAN_NHC_UDP_8BIT_PORT) {