iwlwifi: pcie: Enable MSI mode when using MSI interrupts

On some of the chipsets MSI & INTA interrupts are disabled by default in
the HW registers, and need to be explicitly enabled to be used.

In case MSI-X isn't used, make sure MSI mode is enabled by setting
the relevant HW register.

Signed-off-by: Ido Yariv <idox.yariv@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>