sfc: Limit scope of a Falcon A1 IRQ workaround

We unconditionally acknowledge legacy interrupts just before disabling
them.  This workaround is needed on Falcon A1 but probably not on
later chips where the legacy interrupt mechanism is different.  It was
also originally done after the IRQ handler was removed, not before.
Restore the original behaviour for Falcon A1 only by doing this
acknowledgement in the efx_nic_type::fini operation.

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