sfc: suppress ENOENT error messages from MC_CMD_MAC_STATS

MC_CMD_MAC_STATS can be called on a function before a
vadaptor has been created, as the kernel can call into this
through ndo_get_stats/ndo_get_stats64.

If MC_CMD_MAC_STATS is called before the DMA queues have been
setup, so that a vadaptor has not been created yet, firmware
will return ENOENT. This is expected, so suppress the MCDI
error message in this case.

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