NVMe: Return correct value from interrupt handler

The interrupt handler currently reports whether it found any new
completion queue entries.  If the completion queue is primarily being
processed by a method other than the interrupt handler, it may return
IRQ_NONE so often that Linux thinks that the interrupt is being falsely
triggered.

To solve this problem, report whether any completion queue entries have
been seen since the last interrupt was received for this queue.

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