sfc: Change BUG_ON to WARN_ON and recovery code.

Pre-emptively fix a checkpatch warning in a subsequent patch.

Signed-off-by: Jonathan Cooper <jonathan.s.cooper@amd.com>
Co-developed-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>