staging: vt6656: avoid discarding called function's return code

Change some of the driver's functions in order to handle error codes
instead of discarding them. These function now returns 0 on success and
a negative errno value on error.

Signed-off-by: Quentin Deslandes <quentin.deslandes@itdev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>