sfc: Preserve rx_frm_trunc counters when resizing DMA rings

We allocate efx_channel structures with kzalloc() so we don't need to
zero-initialise individual fields in efx_probe_channel().  Further,
this function will be called again during DMA ring resizing and we
should not reset any statistics then.

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