hw/net/vmxnet3: Fix guest-triggerable assert()

hw/net/vmxnet3: Fix guest-triggerable assert()

The assert() that checks for valid MTU sizes can be triggered by
the guest (e.g. with the reproducer code from the bug ticket
https://gitlab.com/qemu-project/qemu/-/issues/517 ). Let's avoid
this problem by simply logging the error and refusing to activate
the device instead.

Fixes: d05dcd94ae ("net: vmxnet3: validate configuration values during activate")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
[Mjt: change format specifier from %d to %u for uint32_t argument]