iwlwifi: pcie: read the interrupt cause from the handler

iwlwifi: pcie: read the interrupt cause from the handler

We now disable the interrupts in the hardware from the
upper half and all the rest (including reading the interrupt
cause) is done in the handler.

Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>