iio: rradc: Update reading BATT_ID channel

There is a need for RRADC driver to return fresh results
of the BATT_ID channel which could take upto 1.5 seconds
in round robin mode. Switch the RRADC driver to continuous
mode when client requests the BATT_ID channel to obtain
the results faster. In addition explicity force the batt_id
to ensure it forces a conversion on the new read. The
existing USBIN_V channel also requires having the RRADC in
continuous mode therefore have a common support for clients
that uses continuous mode.

Change-Id: I5b89ed9f0015bc6db3adce22e3ac6d0d0283e2bf
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>