hwmon: qpnp-adc-voltage: Assign vadc_dev before registering TZ

The vadc_dev is used in qpnp_vadc_read() when registering the thermal
zone device. The NULL value would cause the qpnp_vadc_read() failed by
returning -EPROBE_DEFER or cause a kernel crash.

Change-Id: I47b88da9dc39bd3df8678a734206b29912b83718
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>