net: xilinx_emacps: Fix wrong udp checksum offload calculation

When the UDP checksum is offloaded, if the usb header checksum field is
non zero then the udp checksum calculated by the hw is wrong. So, the
driver should ensure that the checksum field should be zero.

Signed-off-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>