net: ravb: Disable IP header TX checksum offloading

net: ravb: Disable IP header TX checksum offloading

For IPv4 packets, the header checksum will always be calculated in software
in the TX path (Documentation/networking/checksum-offloads.rst says "No
offloading of the IP header checksum is performed; it is always done in
software.") so there is no advantage in asking the hardware to also
calculate this checksum.

Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>