iio: mcp320x: Fix NULL pointer dereference

On reading in_voltage_scale of we got an NULL pointer dereference Oops.

The reason for this is, that mcp320x_read_raw tries to access
chip_info->resolution from struct mcp320x, but chip_info is never set.

chip_info was never set since the driver was added, but there was no
acute problem, because it was not referenced.
The acute problem exists since
b12206e9
iio: adc: mcp320x. Add support for more ADCs

This patch fixes the issue by setting chip_info in mcp320x_probe.

Signed-off-by: Manfred Schlaegl <manfred.schlaegl@gmx.at>
Reviewed-by: Michael Welling <mwelling@ieee.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>