iwlwifi: pcie: Configure shared interrupt vector in MSIX mode

In case the OS provides fewer interrupts than requested, different
causes will share the same interrupt vector as follow:
1.One interrupt less: non rx causes shared with FBQ.
2.Two interrupts less: non rx causes shared with FBQ and RSS.
3.More than two interrupts: we will use fewer RSS queues.

Also make the request depend on the number of online CPUs
instead of possible CPUs.

Signed-off-by: Haim Dreyfuss <haim.dreyfuss@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>