can: ctucanfd: fix SSP_SRC in cases when bit-rate is higher than 1 MBit.

The Secondary Sample Point Source field has been
set to an incorrect value by some mistake in the
past

0b01 - SSP_SRC_NO_SSP - SSP is not used.

for data bitrates above 1 MBit/s. The correct/default
value already used for lower bitrates is

0b00 - SSP_SRC_MEAS_N_OFFSET - SSP position = TRV_DELAY
(Measured Transmitter delay) + SSP_OFFSET.

The related configuration register structure is described
in section 3.1.46 SSP_CFG of the CTU CAN FD
IP CORE Datasheet.

The analysis leading to the proper configuration
is described in section 2.8.3 Secondary sampling point
of the datasheet.

The change has been tested on AMD/Xilinx Zynq
with the next CTU CN FD IP core versions:

- 2.6 aka master in the "integration with Zynq-7000 system" test
6.12.43-rt12+ #1 SMP PREEMPT_RT kernel with CTU CAN FD git
driver (change already included in the driver repo)
- older 2.5 snapshot with mainline kernels with this patch
applied locally in the multiple CAN latency tester nightly runs
6.18.0-rc4-rt3-dut #1 SMP PREEMPT_RT
6.19.0-rc3-dut

The logs, the datasheet and sources are available at

https://canbus.pages.fel.cvut.cz/

Signed-off-by: Ondrej Ille <[e-mail]>
Signed-off-by: Pavel Pisa <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Fixes: 2dcb8e8782d8 ("can: ctucanfd: add support for CTU CAN FD open-source IP core - bus independent part.")
Cc: [e-mail]
Signed-off-by: Marc Kleine-Budde <[e-mail]>

drivers/net/can/ctucanfd/ctucanfd_base.c

···
 		}
 
 		ssp_cfg = FIELD_PREP(REG_TRV_DELAY_SSP_OFFSET, ssp_offset);
-		ssp_cfg |= FIELD_PREP(REG_TRV_DELAY_SSP_SRC, 0x1);
+		ssp_cfg |= FIELD_PREP(REG_TRV_DELAY_SSP_SRC, 0x0);
 	}
 
 	ctucan_write32(priv, CTUCANFD_TRV_DELAY, ssp_cfg);