iwlwifi: pcie: fix emergency path

Allocator swaps the pending requests with 0 when it starts
working. This means that relying on it n RX path to decide if
to move to emergency is not always a good idea, since it may
be zero, but there are still a lot of unallocated RBs in the
system. Change allocator to decrement the pending requests on
real time. It is more expensive since it accesses the atomic
variable more times, but it gives the RX path a better idea
of the system's status.

Reported-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Fixes: 868a1e86 ("iwlwifi: pcie: avoid empty free RB queue")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>