staging: bcm2835-camera: Fix warnings about string ops on v4l2 uapi.

The v4l2 uapi uses u8[] for strings, so cast those to char * to avoid
compiler warnings about unsigned vs signed with sprintf() and friends.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>