iio: adis16400: Fix burst transfer for adis16448

The adis16448, unlike the other chips in this family, in addition to the
hardware channels also sends out the DIAG_STAT register in burst mode
before them. Handle that case by skipping over the first 2 bytes before we
pass the received data to the buffer.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Fixes: 76ada52f ("iio:adis16400: Add support for the adis16448")
Cc: <stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>