V4L/DVB (11607): soc-camera: add a free_bus method to struct soc_camera_link

Currently pcm990 camera bus-width management functions request a GPIO and never
free it again. With this approach the GPIO extender driver cannot be unloaded
once camera drivers have been loaded, also unloading theb i2c-pxa bus driver
produces errors, because the GPIO extender driver cannot unregister properly.
Another problem is, that if camera drivers are once loaded before the GPIO
extender driver, the platform code marks the GPIO unavailable and only a reboot
helps to recover. Adding an explicit free_bus method and using it in mt9m001
and mt9v022 drivers fixes these problems.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>