machine: read factory MAC on SWTG018AS-A V2.0

Horaco HC-SWTGW218AS units on this PCB keep their factory MAC at
0x1FC000, the offset the other boards in machine/ read it from, so use
it instead of a generated locally administered one. If the flash holds
no valid MAC there, the generated one is still used.

machine/SWTG018AS_A_V_2_0.c

@@ -2,6 +2,7 @@
 
 __code const struct machine machine = {
 	MACHINE_COMMON_SETTING,
+	.mac_flash_offset = 0x1FC000,
 	.n_sfp = 1,
 	.log_to_phys_port = {1, 2, 3, 4, 5, 6, 7, 8, 9},
 	.phys_to_log_port = {0, 1, 2, 3, 4, 5, 6, 7, 8},