can: tcan4x5x: fix data length in regmap write path

In regmap_spi_gather_write() the "addr" is prepared. The chip expects
the number of 32 bit words to write in the lower 8 bits of addr. However
the number of byte to write in shifted left by 3 (== divided by 8).

The function tcan4x5x_regmap_write() is called with a data buffer, which
holds the register information in the first 32 bits, followed by the
actual data. tcan4x5x_regmap_write() calls regmap_spi_gather_write()
with the val pointer pointing to the actual data (i.e. the original
pointer is incremented by 4 bytes), but without decrementing the count.

If the regmap framework only calls tcan4x5x_regmap_write() to read
single 32 bit registers these two bugs cancel each other.

This patch fixes the code.

Fixes: 5443c226 ("can: tcan4x5x: Add tcan4x5x driver to the kernel")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>