iwlwifi: pcie: prefer to load the firmware in one shot

Users complained about allocation failures, so we loaded
the firmware in small chunks (PAGE_SIZE). This makes the
firmware restart considerably slower.
So, always prefer to load it in one shot allocating a big
chunk of coherent, and use smaller chunks as a fallback
solution.

On my laptop, this reduces the fw loading time from 120ms
to 20ms.

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