net/hns3: add restriction on setting VF MTU

net/hns3: add restriction on setting VF MTU

When Rx of scattered packets is off, we have some possibility of using
vector Rx process function or simple Rx functions in hns3 PMD driver.
If the input MTU is increased and the maximum length of received packets
is greater than the length of a buffer for Rx packets, the hardware
network engine needs to use multiple BDs and buffers to store these
packets. This will cause problems when still using vector Rx process
function or simple Rx function to receiving packets. So, when Rx of
scattered packets is off and device is started, it is not permitted to
increase MTU so that the maximum length of Rx packets is greater than Rx
buffer length.

Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>