hwmon: qpnp-adc-voltage: Update condition for reading temp through VADC

ADC driver prints error log whenever vadc read fails. Update the
condition such that log gets printed for error case and not for probe
deferal.

Change-Id: Ieff7f82fbe508a8422f666635a489f45f2deb41f
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>