Spurious PFVF interrupts can happen when either the ISR is invoked
without a valid source being set or, otherwise, when no interrupt bit
is set in the PFVF register containing the message.

The latter test was present for GEN2 devices but missing for GEN4, this
patch fills the gap.
Signed-off-by: Marco Chiappero <marco.chiappero@intel.com>
Reviewed-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>