NVMe: Only clear the enable bit when disabling controller

Many of the bits in the Controller Configuration register may only be
modified when the Enable bit is clear.  Clearing them at the same time
as the Enable bit might be OK, but let's play it safe and only touch the
Enable bit.

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