net/mlx5e: Unify logic of MTU boundaries

Expose a new helper that wraps the logic for setting the
netdevice's MTU boundaries.
Use it for the different components (Eth, rep, IPoIB).

Set the netdevice min MTU to ETH_MIN_MTU, and the max according
to both the FW capability and the kernel definition.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>