can: calc_bittiming: replace misleading "nominal" by "reference"

can: calc_bittiming: replace misleading "nominal" by "reference"

The functions can_update_sample_point() and can_calc_bittiming() are
generic and meant to be used for both the nominal and the data bittiming
calculation.

However, those functions use misleading terminologies such as "bitrate
nominal" or "sample point nominal". Replace all places where the word
"nominal" appears with "reference" in order to better distinguish it from
the calculated values.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://patch.msgid.link/20251126-canxl-v8-12-e7e3eb74f889@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>