nvme_drv: only wait for first admin cmd

Leave the loop if we processed the cmd we were looking for. That
should fix the delay users notice in Sculpt when NVMe is used.

Issue #3369.

repos/os/src/drivers/nvme/main.cc

@@ -817,6 +817,7 @@ struct Nvme::Controller : public Genode::Attached_mmio
 			success = true;
 
 			write<Admin_cdb::Cqh>(_admin_cq.head);
+			break;
 		}
 
 		return success;