bcm2835-v4l2: Fix buffer overflow problem

bcm2835-v4l2: Fix buffer overflow problem

https://github.com/raspberrypi/linux/issues/1447
port_parameter_get() failed to account for the header
(u32 id and u32 size) in the size before memcpying
the response into the response buffer, so overrunning
the provided buffer by 8 bytes.

Account for those bytes, and also a belt-and-braces
check to ensure we never copy more than *value_size
bytes into value.

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