sfc: Make struct efx_special_buffer less special

On EF10, the firmware is in charge of allocating buffer table entries.
Change struct efx_special_buffer to use a struct efx_buffer member,
so that it can be used with efx_nic_{alloc,free}_buffer() in that
case.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>