iio: ti_am335x_adc: Fix: Use same step id at FIFOs both ends

Since AI lines could be selected at will (linux-3.11) the sending
and receiving ends of the FIFO does not agree about what step is used
for a line. It only works if the last lines are used, like 5,6,7,
and fails if ie 2,4,6 is selected in DT.

Signed-off-by: Jan Kardell <jan.kardell@telliq.com>
Tested-by: Zubair Lutfullah <zubair.lutfullah@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: stable@vger.kernel.org