spi: spi-fsl-lpspi: fix watermark truncation caused by type cast

spi: spi-fsl-lpspi: fix watermark truncation caused by type cast

't->len' is an unsigned integer, while 'watermark' and 'txfifosize' are
u8. Using min_t with typeof(watermark) forces both values to be cast to
u8, which truncates len when it exceeds 255. For example, len = 4096
becomes 0 after casting, resulting in an incorrect watermark value.

Use a wider type in min_t to avoid truncation and ensure the correct
minimum value is applied.

Fixes: a750050349ea ("spi: spi-fsl-lpspi: use min_t() to improve code")
Signed-off-by: Carlos Song <carlos.song@nxp.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://patch.msgid.link/20251117030355.1359081-1-carlos.song@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>