iwlwifi: pcie: disable interrupts upon PCIe alloc

In case RFKILL is in KILL position, the NIC will issue an
interrupt straight away. This interrupt won't be sent
because it is masked in the hardware.
But if our interrupt service routine is called for another
reason (SHARED_IRQ), then we'll look at the interrupt cause
and service it. This can cause bad things if we are not
ready yet.
Explicitly clean the interrupt cause register to make sure
we won't service anything before we are ready to.

Cc: <stable@vger.kernel.org> [3.14]
Reported-and-tested-by: Alexander Monakov <amonakov@gmail.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>