mem: Fix a bug in the DRAM controller arbitration
mem: Fix a bug in the DRAM controller arbitration

Fix a minor issue that affects multi-rank systems.
(transplanted from 471d390943f0a7f3cf1574e282e0158924fb3e6a)

andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin