colibri_t30: fix usb ethernet functionality

Since commit aa7a6487
("net: Stop including NFS overhead in defragment max") the following
has been reproducibly observed while trying to transfer data over TFTP:

Load address: 0x80408000
Loading: EHCI timed out on TD - token=0x8008d80
T EHCI timed out on TD - token=0x88008d80
Rx: failed to receive: -5

This patch fixes this by upping our maximal de-fragmentation aka IP
packet size again.
Signed-off-by: NMarcel Ziswiler <marcel.ziswiler@toradex.com>

include/configs/colibri_t30.h

@@ -48,6 +48,7 @@
 
 /* General networking support */
 #define CONFIG_IP_DEFRAG
+#define CONFIG_NET_MAXDEFRAG		(16384 + 4096 + 24)
 #define CONFIG_TFTP_BLOCKSIZE		16384
 #define CONFIG_TFTP_TSIZE