i2c: riic: Introduce a separate variable for IRQ

Refactor the IRQ handling in riic_i2c_probe by introducing a local variable
`irq` to store IRQ numbers instead of assigning them to `ret`. This change
improves code readability and clarity.

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