staging: hikey9xx: hi6421v600-regulator: fix delay logic

staging: hikey9xx: hi6421v600-regulator: fix delay logic

The original driver, which can be seen at
commit 42f24d9d446a ("staging: regulator: add a regulator driver for HiSilicon 6421v600 SPMI PMIC")
had a complex logic to ensure that there won't be multiple power
enable/disable commands running at the same time. At the original
logic, it were ensured that:

- a next power up/down would wait for at least the on/off period;
- an extra delay would be granted. It turns that such extra delay
  has a value of zero, but it was relying on gettimeofday()
  call, which can take some time.

This was later simplified, but there are still some possible
issues. In order to avoid that, let's simply add a delay
to wait for the power up line to stabilize after powering up
a device.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/6733dac9813ba6688def404142cb7b964accf758.1611212783.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>