media: bcm2835-unicam: Reinstate V4L2_CAP_READWRITE in the caps

v4l2-compliance throws a failure if the device doesn't advertise
V4L2_CAP_READWRITE but allows read or write operations.
We do support read, so reinstate the flag.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>