serial: sc16is7xx: add check for unsupported SPI modes during probe

The original comment is confusing because it implies that variants other
than the SC16IS762 supports other SPI modes beside SPI_MODE_0.

Extract from datasheet:
    The SC16IS762 differs from the SC16IS752 in that it supports SPI clock
    speeds up to 15 Mbit/s instead of the 4 Mbit/s supported by the
    SC16IS752... In all other aspects, the SC16IS762 is functionally and
    electrically the same as the SC16IS752.

The same is also true of the SC16IS760 variant versus the SC16IS740 and
SC16IS750 variants.

For all variants, only SPI mode 0 is supported.

Change comment and abort probing if the specified SPI mode is not
SPI_MODE_0.

Fixes: 2c837a8 ("sc16is7xx: spi interface is added")
Cc:  <[email protected]>
Signed-off-by: Hugo Villeneuve <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: hvilleneuvedoo

drivers/tty/serial/sc16is7xx.c

@@ -1733,7 +1733,10 @@ static int sc16is7xx_spi_probe(struct spi_device *spi)
 
 	/* Setup SPI bus */
 	spi->bits_per_word	= 8;
-	/* only supports mode 0 on SC16IS762 */
+	/* For all variants, only mode 0 is supported */
+	if ((spi->mode & SPI_MODE_X_MASK) != SPI_MODE_0)
+		return dev_err_probe(&spi->dev, -EINVAL, "Unsupported SPI mode\n");
+
 	spi->mode		= spi->mode ? : SPI_MODE_0;
 	spi->max_speed_hz	= spi->max_speed_hz ? : 15000000;
 	ret = spi_setup(spi);