sfc: don't free_irq()s if they were never requested

If efx_nic_init_interrupt fails, or was never run (e.g. due to an earlier
 failure in ef100_net_open), freeing irqs in efx_nic_fini_interrupt is not
 needed and will cause error messages and stack traces.
So instead, only do this if efx_nic_init_interrupt successfully completed,
 as indicated by the new efx->irqs_hooked flag.

Fixes: 965b549f ("sfc_ef100: implement ndo_open/close and EVQ probing")
Signed-off-by: Edward Cree <ecree@solarflare.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>