leds: qpnp-wled: enable VREF_UP in TEST4 register

False ovp (over voltage protection) can be triggered
for LCD panels when the brightness is low. Avoid this
by enabling VREF_UP in TEST4 register.

Change-Id: I5c503f2c806499fd565234757fdac87367951f60
Signed-off-by: Mohan Pallaka <mpallaka@codeaurora.org>