staging: vt6655: extract index manupulation out of function call

This patch will remove if/else by selecting proper argument before
function call, also index is updated before function call.

Signed-off-by: Matej Dujava <mdujava@kocurkovo.cz>
Link: https://lore.kernel.org/r/1588512552-12297-6-git-send-email-mdujava@kocurkovo.cz


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