staging: media: atomisp: drop check for reentrant .s_stream()

The subdev .s_stream() operation shall not be called to start streaming
on an already started subdev, or stop streaming on a stopped subdev.
Remove the check that guards against that condition.

Also fixes a smatch warning:

drivers/staging/media/atomisp/i2c/atomisp-gc0310.c:446 gc0310_s_stream() warn: missing error code 'ret'

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
CC: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>