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: Ben Hutchings <bhutchings@solarflare.com>