i2c: riic: Use GENMASK() macro for bitmask definitions

Replace raw bitmask values with the `GENMASK()` macro in the `i2c-riic`
driver to improve readability and maintain consistency.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>