hwmon: qpnp-adc: Update support to read USB_IN_V on PMI632

With existing support to read USB_IN_V, sometimes two consecutive
read requests for REG_GND and USB_IN_V are queued, leading to
REF_GND channel being read instead of USB_IN_V. Add delay to
ensure end of conversion occurs for reading REF_GND before
sending conversion request for USB_IN_V channel.

Change-Id: I511f64e1cc1f057655eed484f4a39b63fb14d017
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>
Signed-off-by: Siva Kumar Akkireddi <sivaa@codeaurora.org>