tty: serial: samsung: use u32 for register interactions

All registers of the IP have 32 bits. Use u32 variables when reading
or writing from/to the registers. The purpose of those variables becomes
clearer.
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Link: https://lore.kernel.org/r/20240119104526.1221243-9-tudor.ambarus@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>