fsl_i2c: Impl. AN2919, rev 5 to calculate FDR/DFSR

The latest AN2919 has changed the way FDR/DFSR should be calculated.
Update the driver according to spec. However, Condition 2
is not accounted for as it is not clear how to do so.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Acked-by: Wolfgang Grandegger <wg@grandegger.com>