sfc: Fix null function pointer in efx_sriov_channel_type

Commit c31e5f9f ('sfc: Add channel specific receive_skb handler and
post_remove callback') added the function pointer field
efx_channel_type::post_remove and an unconditional call through it.

This field should have been initialised to efx_channel_dummy_op_void
in the existing instances of efx_channel_type, but this was only done
in efx_default_channel_type.  Consequently, if a device has SR-IOV
enabled then removing the driver or device will result in an oops.
Signed-off-by: NBen Hutchings <bhutchings@solarflare.com>

drivers/net/ethernet/sfc/siena_sriov.c

@@ -1035,6 +1035,7 @@ efx_sriov_get_channel_name(struct efx_channel *channel, char *buf, size_t len)
 static const struct efx_channel_type efx_sriov_channel_type = {
 	.handle_no_channel	= efx_sriov_handle_no_channel,
 	.pre_probe		= efx_sriov_probe_channel,
+	.post_remove		= efx_channel_dummy_op_void,
 	.get_name		= efx_sriov_get_channel_name,
 	/* no copy operation; channel must not be reallocated */
 	.keep_eventq		= true,