mirror: show the configured monitor port in "mirror status"

"mirror status" printed the local mirroring_port, which is still 0 at
that point, so it named the same port whatever was configured. Print
the port from RTL837x_MIRROR_CTRL instead, which the function has
already read to tell whether mirroring is enabled. send_mirror() decodes
the register the same way for the web interface.

cmd_parser.c

@@ -826,7 +826,7 @@ void parse_mirror(void)
 			print_string("NOT Enabled: ");
 		}
 		print_string("Mirroring port: ");
-		print_phys_port(mirroring_port);
+		print_phys_port(mPort >> 1);
 		reg_read_m(RTL837x_MIRROR_CONF);
 		uint16_t m = sfr_data[0];
 		m = (m << 8) | sfr_data[1];