mfd: Correct revision display for WM1811 revision D

As WM1811 revision C was transparent to software the revision IDs for
subsequent revisions are one less than they would normally be. Correct
for this in log messages.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>