rtl8139: honor large send MSS value

rtl8139: honor large send MSS value

The Large-Send Task Offload Tx Descriptor (9.2.1 Transmit) has a
Large-Send MSS value where the driver specifies the MSS. See the
datasheet here:
http://realtek.info/pdf/rtl8139cp.pdf

The code ignores this value and uses a hardcoded MSS of 1500 bytes
instead. When the MTU is less than 1500 bytes the hardcoded value
results in IP fragmentation and poor performance.

Use the Large-Send MSS value to correctly size Large-Send packets.

Jason Wang <jasowang@redhat.com> noticed that the Large-Send MSS value
mask was incorrect so it is adjusted to match the datasheet and Linux
8139cp driver.

This issue was discussed in the past here:
https://lore.kernel.org/all/20161114162505.GD26664@stefanha-x1.localdomain/

Reported-by: Russell King - ARM Linux <linux@armlinux.org.uk>
Reported-by: Tobias Fiebig <tobias+git@fiebig.nl>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1312
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20221117165554.1773409-4-stefanha@redhat.com>