NVMe: Use pci_(request|release)_mem_regions

Now that we do have pci_request_mem_regions() and pci_release_mem_regions()
at hand, use it in the NVMe driver.

Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
CC: Keith Busch <keith.busch@intel.com>
CC: Jens Axboe <axboe@fb.com>