OMAP4: Fix -EINVAL for vana, vcxio, vdac

OMAP4: Fix -EINVAL for vana, vcxio, vdac

Fixed regulators in twl6030 do not have set_voltage hook.
Regulator core returns -22 if set_voltage is NULL and apply_uV is set
while applying the constraint to set voltage resulting in failure during probe
of these regulators.
Do not set apply_uV for fixed regulators which don't have set_voltage.

machine_constraints_voltage: VANA: failed to apply 2100000uV constraint
twl_reg twl_reg.43: can't register VANA, -22
twl_reg: probe of twl_reg.43 failed with error -22
machine_constraints_voltage: VCXIO: failed to apply 1800000uV constraint
twl_reg twl_reg.44: can't register VCXIO, -22
twl_reg: probe of twl_reg.44 failed with error -22
machine_constraints_voltage: VDAC: failed to apply 1800000uV constraint
twl_reg twl_reg.45: can't register VDAC, -22
twl_reg: probe of twl_reg.45 failed with error -22

Signed-off-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>