iwlwifi: pcie: replace possible_cpus() with online_cpus() in MSIX mode

In MSIX mode the number of irq depends on the number of
possible cpus existing on the host.
This cause to bug in case there are offline cores.
Take into account only the online CPUs instead.
Also save it in temporary variable.

Fixes: commit 2e5d4a8f ("iwlwifi: pcie: Add new configuration to enable MSIX")
Signed-off-by: Haim Dreyfuss <haim.dreyfuss@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>