OMAP3+: voltage: update nominal voltage in voltdm_scale() not VC post-scale

Currently, the nominal voltage is updated in the VC post-scale function
which is common to both scaling methods.  However, this has readabiliy
problems as this update is not where it might be expected.  Instead, move
the updated into voltdm_scale() upon a successful return of voltdm->scale()

Signed-off-by: Kevin Hilman <khilman@ti.com>