staging: vt6656: Use DIV_ROUND_UP macro instead of specific code

Use DIV_ROUND_UP macro instead of specific code with the same purpose.
Also, remove the unused variables.

Signed-off-by: Oscar Carter <oscar.carter@gmx.com>
Reviewed-by: Quentin Deslandes <quentin.deslandes@itdev.co.uk>
Link: https://lore.kernel.org/r/20200326175902.14467-1-oscar.carter@gmx.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>