iwlwifi: pcie: re-organize the PCIe ISR code

Separate the code that simply disables interrupt in the
hardware and the code that checks what interrupt fired.
This will be useful to move the second part in the threaded
handler which will be done in a future patch.

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