mlxsw: spectrum: Set more accurate values for netdevice min/max MTU

Currently, the driver uses ETH_MAX_MTU as maximum MTU of netdevices,
instead, use the accurate value which is supported by the driver.
Subtract Ethernet headers which are taken into account by hardware for
MTU checking, as described in the previous patch.

Set minimum MTU to ETH_MIN_MTU, as zero MTU is not really supported.

With this change:
a. The stack will do the MTU checking, so we can remove it from the driver.
b. User space will be able to query the actual MTU limits.
   Before this patch:
   $ ip -j -d link show dev swp1 | jq | grep mtu
    "mtu": 1500,
    "min_mtu": 0,
    "max_mtu": 65535,

   With this patch:
   $ ip -j -d link show dev swp1 | jq | grep mtu
    "mtu": 1500,
    "min_mtu": 68,
    "max_mtu": 10218,

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/be8232e38c196ecb607f82c5e000ea427ce22abb.1718275854.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>