iwlwifi: don't panic in error path on non-msix systems

The driver uses msix causes-register to handle both msix and non msix
interrupts when performing sync nmi.  On devices that do not support
msix this register is unmapped and accessing it causes a kernel panic.

Solve this by differentiating the two cases and accessing the proper
causes-register in each case.

Reported-by: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Shahar S Matityahu <shahar.s.matityahu@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>