power: smb5: Fix reading of USB VOLTAGE_NOW

Reading USBIN VOLTAGE_NOW when USBIN is not present gives rise to the
following errors:

smblib_get_prop_usb_voltage_now: Failed to read USBIN over vadc, rc=-61
smblib_get_prop_usb_voltage_now: Couldn't write ADC config rc=-61

Fix them.

Fixes: b1c7f424
Change-Id: Idebc1a3c26c179a078cd651ba9a6fd62023b8ed3
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>