I still get the error though.
Instead of flashing by modifying pinmux, I used devmem2 to temporarily change the register value.
when checking dmesg, the following error appears.
[ 280.340212] spi-tegra124-slave 3230000.spi: Tx is not supported in mode 0
[ 280.340215] spi-tegra124-slave 3230000.spi: spi can not start transfer, err -22
[ 280.340301] spi_master spi2: failed to transfer one message from queue