staging: vt6656: Check the return value of vnt_control_out_* calls

Check the return value of vnt_control_out_* function calls. When
necessary modify the function prototype to be able to return the new
checked error code.

It's safe to modify all the function prototypes without fix the call
because the only change is the return value from void to int. If before
the call didn't check the return value, now neither.

Signed-off-by: Oscar Carter <oscar.carter@gmx.com>
Link: https://lore.kernel.org/r/20200425134257.4502-2-oscar.carter@gmx.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>