regmap: spi: Fix potential off-by-one when calculating reserved size

If we ever meet a hardware that uses weird register bits and padding,
we may end up in off-by-one error since x/8 + y/8 might not be equal
to (x + y)/8 in some cases.

bits    pad   x/8+y/8 (x+y)/8
4..7    0..3    0       0 // x + y from 4 up to 7
4..7    4..7    0       1 // x + y from 8 up to 11
4..7    8..11   1       1 // x + y from 12 up to 15
8..15   0..7    1       1 // x + y from 8 up to 15
8..15   8..15   2       2 // x + y from 16 up to 23

Fix this by using (x+y)/8.
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://msgid.link/r/20240605205315.19132-1-andy.shevchenko@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/base/regmap/regmap-spi.c

@@ -122,8 +122,7 @@ static const struct regmap_bus *regmap_get_spi_bus(struct spi_device *spi,
 			return ERR_PTR(-ENOMEM);
 
 		max_msg_size = spi_max_message_size(spi);
-		reg_reserve_size = config->reg_bits / BITS_PER_BYTE
-				 + config->pad_bits / BITS_PER_BYTE;
+		reg_reserve_size = (config->reg_bits + config->pad_bits) / BITS_PER_BYTE;
 		if (max_size + reg_reserve_size > max_msg_size)
 			max_size -= reg_reserve_size;