media: atomisp: Simplify atomisp_[start|stop]_streaming()

Now that continuous mode is gone and we only have 1 /dev/video# node,
the videobuf2 core guarantees that atomisp_[start|stop]_streaming() will
only be called one at a time.

So there is no need to check for things like the isp is already streaming
when starting, or other streams still being active while stopping.
Remove checks for these from atomisp_[start|stop]_streaming().

While at it also improve the logging a bit:
1. Remove the dev_err(isp->dev, "atomisp_reset") logged on every
   stream stop (even though everything is fine.
2. Log a message when starting/stopping the sensor stream fails

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>