iwlwifi: pcie: don't read INTA register in ICT IRQ handler

There's no reason to read the INTA register in the ICT IRQ
handler, this interrupt mechanism is designed to not have
to read as many registers as the regular one. Not reading
the INTA register gives a significant performance/CPU use
improvement.

Since we still want to get this info, fetch it only if
the ISR debug level is enabled.

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