i2c: riic: Use predefined macro and simplify clock tick calculation

Replace the hardcoded `1000000000` with the predefined `NSEC_PER_SEC`
macro for clarity. Simplify the code by introducing a `ns_per_tick`
variable to store `NSEC_PER_SEC / rate`, reducing redundancy and
improving readability.

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