VUSB is a fixed regulator, so get rid of the apply_uV constraint
for it, which fixes the following error seen at boot on omap4
SDP and PANDA boards.

machine_constraints_voltage: VUSB: failed to apply 3300000uV constraint
twl_reg twl_reg.46: can't register VUSB, -22
twl_reg: probe of twl_reg.46 failed with error -22
Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>