DHCP: Add parentheses to macro definitions

Missing parentheses in IP_UDP_SIZE caused wrong computation of MSZ
(option57). When mtu is 1500, current MSZ value is 1488=1500-20+8, while
the correct value should be 1472=1500-(20+8).

Also added parentheses for BOOTP_MIN_MTU.

Author: acst1223

src/dhcp.c

@@ -98,8 +98,8 @@
 __CTASSERT(sizeof(struct ip)		== 20);
 __CTASSERT(sizeof(struct udphdr)	== 8);
 __CTASSERT(sizeof(struct bootp)		== 300);
-#define IP_UDP_SIZE	sizeof(struct ip) + sizeof(struct udphdr)
-#define BOOTP_MIN_MTU	IP_UDP_SIZE + sizeof(struct bootp)
+#define IP_UDP_SIZE	(sizeof(struct ip) + sizeof(struct udphdr))
+#define BOOTP_MIN_MTU	(IP_UDP_SIZE + sizeof(struct bootp))
 
 struct dhcp_op {
 	uint8_t value;