[PATCH] nvme: add wrapper for nvme ctrl register r/w

[Sagi Grimberg]

>> Simplify nvme ctrl register r/w code.
>>
>> Signed-off-by: weiping zhang <zhangweiping at didichuxing.com>
> 
> This seems generally useful, especially with more consolidation
> of code to the core.  Sagi - do you want to integrate this into
> your consolidation series and fixup any clashes?

Not sure whats special about register read/write that they
deserve a wrapper, but its easy enough to get in before
my patches if I'll see a review on it.