drivers: ethernet: xlnx_gem: invert RX/TX checksum offload behaviour

Invert the RX/TX checksum offloading to hardware behaviour so that
hardware checksum generation / evaluation for TCP/UDP packets on
either IPv4 or IPv6 is enabled by default, but can be disabled
explicitly.

This behaviour, which should optimize network performance, has
become possible with the implementation of the device driver's
get_config function, indicating to the network stack that HW
checksum offloading is not supported for, e.g., ICMP packets.
Before this implementation, enabling the HW checksum offloading
resulted in invalid packets for any unsupported protocol and
could therefore not be enabled by default.

For QEMU, which does not support the emulation of the HW
checksum offloading, automatically disable the offloading.

Signed-off-by: Immo Birnbaum <mail@birnbaum.immo>