qed: Add sanity check for SIMD fastpath handler.

Avoid calling a SIMD fastpath handler if it is NULL. The check is needed
to handle an unlikely scenario where unsolicited interrupt is destined to
a PF in INTa mode.

Fixes: fe56b9e6 ("qed: Add module with basic common support")
Signed-off-by: Sudarsana Reddy Kalluru <Sudarsana.Kalluru@cavium.com>
Signed-off-by: Ariel Elior <ariel.elior@cavium.com>
Signed-off-by: Michal Kalderon <Michal.Kalderon@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>