can: dev: remove unused variable from can_calc_bittiming() function

this patch removes best_rate variable from can_calc_bittiming()
function which was set but was never used.

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>