iwlwifi: pcie: prepare the enablement of 31 TFD queues

Some devices have 31 TFD queues. Don't enable it yet since
there are still issues with it, but at least prepare the
code for it. There was a bug in the read pointer assignment,
fix that. Also, move the inline functions to iwl-scd.h which
is the right place.

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