Move DHCP out of BANK2 into BANK3

BANK2 outgrew its 48 KB window on PCB_SWTG018AS_V2_1_0 (49182 bytes, 30
over) once the spanning tree code learnt about link aggregation, and the
image builder refuses to link. Only KP_9000_6XHML_X2 is built in CI, so
the overflow was not caught there.

dhcp.c is self-contained: its three exports are __banked, the rest of
the module is only reached from inside it, it holds no __code pointers,
and its uip calls already crossed into BANK1. Moving the pragma is the
whole change. BANK2 49182 -> 43691 on PCB_SWTG018AS_V2_1_0, BANK3 5709
-> 11206, BANK1 +6 for the trampolines; the generated assembly shows no
plain lcall leaving the module's bank.

dhcp.c

@@ -50,7 +50,7 @@ __xdata uip_ipaddr_t server;
 #define DHCP_PARAM_DNS		6
 #define DHCP_END		255
 
-#pragma codeseg BANK2
+#pragma codeseg BANK3
 #pragma constseg BANK2
 
 struct dhcp_pkt {