net: hibmcge: Implement rx_poll function to receive packets

Implement rx_poll function to read the rx descriptor after
receiving the rx interrupt. Adjust the skb based on the
descriptor to complete the reception of the packet.

Signed-off-by: Jijie Shao <shaojijie@huawei.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>