iwlwifi: fix PCIe interrupt handle return value

By accident, commit eb647644
("iwlwifi: protect use_ict with irq_lock") changed the return
value of the iwl_pcie_isr() function in case it handles an
interrupt -- it now returns IRQ_NONE instead of IRQ_HANDLED.

Put back the correct return value.

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