iwlwifi: pcie: fix rb_allocator workqueue allocation

We don't handle failures in the rb_allocator workqueue allocation
correctly. To fix that, move the code earlier so the cleanup is
easier and we don't have to undo all the interrupt allocations in
this case.

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