can: m_can: is_lec_err(): clean up LEC error handling

can: m_can: is_lec_err(): clean up LEC error handling

The PSR register contains among other the error for the CAN
arbitration phase (LEC bits) and CAN data phase (DLEC bits).

Prepare is_lec_err() to be called with the (D)LEC value only instead
of the whole PSR register. While there rename LEC_UNUSED to
LEC_NO_CHANGE to match the latest M_CAN reference manual.

Link: https://lore.kernel.org/all/20221019211611.1605764-1-mkl@pengutronix.de
Reviewed-by: Chandrasekar Ramakrishnan <rcsekar@samsung.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>