iwlwifi: pcie: return inta from iwl_pcie_int_cause_{non_}ict

These functions are meant to return an interrupt cause and
not an irqreturn_t.
We still return IRQ_HANDLED if we had an error and IRQ_NONE
if our device hasn't fired any interrupt.

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