iwlwifi: pcie: rx: use rxq queue_size instead of constant

This is a little less efficient now as it's known to be a
multiqueue device in this function, but a future patch will
have to use a variable here anyway, so use rxq->queue_size
now instead to make it clearer.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>