sfc: Remove EFX_REV_SIENA_A0

The workarounds for bug 8568 and 17213 are no longer needed.

Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Acked-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>