OMAP4: PM: TWL6030: fix uv to voltage for >0x39

using 1.35V as a check is not correct, we know that beyond 0x39,
voltages are non linear - hence use the conversion iff uV greater
than that for 0x39. For example, with  709mV as the smps offset,
the max linear is actually 1.41V(0x39vsel)!

Signed-off-by: Nishanth Menon <nm@ti.com>