iwlwifi: fix PCIe interrupt handle return value

iwlwifi: fix PCIe interrupt handle return value

By accident, commit eb6476441bc2fecf6232a87d0313a85f8e3da7f4
("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: [email protected]
Reviewed-by: Emmanuel Grumbach <[email protected]>
Signed-off-by: Johannes Berg <[email protected]>