net/hns3: remove restriction on setting VF MTU

The hns3 PF/VF devices on the same port share the hardware MTU
configuration. Currently, we send mailbox to inform hns3 PF kernel
ethdev driver to finish hardware MTU configuration in hns3 VF PMD
driver, there is no need to stop the port for hns3 VF device, and
the MTU value issued by hns3 VF PMD driver must be less than or
equal to PF's MTU.

Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>