iwlwifi: pcie: no need to save inta in trans_pcie

This was useful when the handling was not in the same
context as the interrupt cause retrieval: we could have
several hard interrupts until the handler gets called.
Since we retrieve the interrupt cause in the handler itself,
there is no need to OR the interrupt causes.

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