mlxsw: spectrum: Fix misuse of hard_header_len

In order to specify that the mlxsw spectrum driver needs additional
headroom for packets, there have been use of the hard_header_len field of
the netdevice struct.

This commit changes that to use needed_headroom instead, as this is the
correct way to do that.

Fixes: 56ade8fe ("mlxsw: spectrum: Add initial support for Spectrum ASIC")
Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Acked-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>