can: bittiming(): replace open coded variants of can_bit_time()

Commit 1c47fa6b

 ("can: dev: add a helper function to calculate the
duration of one bit") added the helper function can_bit_time().

Replace open coded variants of can_bit_time() by the helper function.

Link: https://lore.kernel.org/all/20230202110854.2318594-2-mkl@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>