staging: bcm2835: mark all symbols as 'static'

I got a link error in allyesconfig:

drivers/staging/media/platform/bcm2835/bcm2835-camera.o: In function `vidioc_enum_framesizes':
bcm2835-camera.c:(.text.vidioc_enum_framesizes+0x0): multiple definition of `vidioc_enum_framesizes'
drivers/media/platform/vivid/vivid-vid-cap.o:vivid-vid-cap.c:(.text.vidioc_enum_framesizes+0x0): first defined here

While both drivers are equally at fault for this problem, the bcm2835 one was
just added and is easier to fix, as it is only one file, and none of its symbols
need to be globally visible. This marks the three global symbols as static.

Fixes: 7b3ad5ab ("staging: Import the BCM2835 MMAL-based V4L2 camera driver.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>