nvme: Respect timeout when en/disabling the controller

So far the driver unconditionally delays 10ms when en/disabling the
controller and still return 0 if 10ms times out. In fact, spec defines
a timeout value in the CAP register that is the worst case time that
host software shall wait for the controller to become ready.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>