i2c: riic: Allow setting frequencies lower than 50KHz

The MR1.CKS field is 3 bits wide and all the possible values (from 0 to
7) are valid. This is true for all the SoCs currently integrated in
upstream Linux. Take into account CKS=7 which allows setting bus
frequencies lower than 50KHz. This may be useful at least for debugging.

Fixes: d982d665 ("i2c: riic: remove clock and frequency restrictions")
Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>