nvme: remove CC register read-back during enabling

Any non-posted read should flush the previous write, so we don't
necessarily need to read back the value we just wrote. I've found at
least some controllers that respond with 0 for short moments after
writing the CC register with EN (enable) cleared, so the read-back is
overwriting our valid ctrl_config value and ends up breaking on the
subsequent enabling.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>