sfc: Initialise MCDI buffers to 0 on declaration.

In order to avoid MC bugs the flags field needs to be set to 0.
Instead of explicitly clearing out the flags individually, a
better way to do this is to memset the MCDI_BUF to 0.

Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>