mlxsw: pci: Remove unused bit

The overrun ignore bit isn't supported by the device's firmware and was
recently removed from the programmer's reference manual (PRM).

Remove it from the driver as well.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>