i3c: mipi-i3c-hci: use parity8 helper instead of open coding it

i3c: mipi-i3c-hci: use parity8 helper instead of open coding it

The kernel has now a generic helper for getting parity with easier to
understand semantics. Make use of it. Here, it also fixes a bug because
the correct algorithm is using XOR ('^=') instead of ADD ('+=').

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Link: https://lore.kernel.org/r/20250107090204.6593-5-wsa+renesas@sang-engineering.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>