wifi: iwlwifi: pcie: refactor RB status size calculation

We have three places doing this check, and even in
slightly different ways (with/without an intermediate).
Refactor that to a new small inline function.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230620125813.f3e87ddd5bce.Ifefba753043b68c394590a35bc6914a0f6497fd3@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>