staging: vt6656: use meaningful error code during buffer allocation

Check on called function's returned value for error and return 0 on
success or a negative errno value on error instead of a boolean value.

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