net/mlx5e: Fix hw mtu initializing at XDP SQ allocation

Current xdp xmit functions logic (mlx5e_xmit_xdp_frame_mpwqe or
mlx5e_xmit_xdp_frame), validates xdp packet length by comparing it to
hw mtu (configured at xdp sq allocation) before xmiting it. This check
does not account for ethernet fcs length (calculated and filled by the
nic). Hence, when we try sending packets with length > (hw-mtu -
ethernet-fcs-size), the device port drops it and tx_errors_phy is
incremented. Desired behavior is to catch these packets and drop them
by the driver.

Fix this behavior in XDP SQ allocation function (mlx5e_alloc_xdpsq) by
subtracting ethernet FCS header size (4 Bytes) from current hw mtu
value, since ethernet FCS is calculated and written to ethernet frames
by the nic.

Fixes: d8bec2b2

 ("net/mlx5e: Support bpf_xdp_adjust_head()")
Signed-off-by: Adham Faris <afaris@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>