NVMe: Handle ioremap failure

Decrement the number of queues required for doorbell remapping until
the memory is successfully mapped for that size.

Additional checks are done so that we don't call free_irq if it has
already been freed.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>