net/ixgbe: fix configuration of max frame size

net/ixgbe: fix configuration of max frame size

For some types of NIC, jumbo frame is not supported in IOV mode,
so if a VF requests to configure the frame size to not bigger
than IXGBE_ETH_MAX_LEN, the kernel driver returns 0, but the DPDK
ixgbe PMD returns -1, this will cause the VF to fail to start
when the PF driven by DPDK ixgbe PMD.

This patch keeps ixgbe PMD's handling mode consistent with kernel
driver in above situation.

In addition, the value set by the command IXGBE_VF_SET_LPE
represents the max frame size, not the mtu.

Fixes: 1b9ea09c067b ("ixgbe: support X550")
Fixes: 95a27b3ba5f5 ("net/ixgbe: enable jumbo frame for VF")
Cc: stable@dpdk.org
Signed-off-by: Alvin Zhang <alvinx.zhang@intel.com>
Acked-by: Jeff Guo <jia.guo@intel.com>