staging: vt6656: Add formula to the vnt_rf_addpower function

Use a formula to calculate the return value of the vnt_rf_addpower
function instead of the "if" statement with literal values for every
case.

Signed-off-by: Oscar Carter <oscar.carter@gmx.com>
Link: https://lore.kernel.org/r/20200425141514.5528-1-oscar.carter@gmx.com


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