iio: ads: avoid i2c transfer after bus shutdown

iio: ads: avoid i2c transfer after bus shutdown

Addig shutdown call back to ignore i2c transfer to
read/write ads chip registers to get channel value
after i2c bus shutdown.

Bug 200100664

Change-Id: If6304910ce7685d465aeba8e6c6808d947ee017c
Signed-off-by: Venkat Reddy Talla <vreddytalla@nvidia.com>
Reviewed-on: http://git-master/r/736580
(cherry picked from commit 72d591a6f125afc775d3f7dc5878646f6e604f6d)
Reviewed-on: http://git-master/r/738457
Reviewed-by: Laxman Dewangan <ldewangan@nvidia.com>
Tested-by: Laxman Dewangan <ldewangan@nvidia.com>