staging:iio:adis16130: Fix sample reading

The driver first does a proper read of the data register, but right after that
it does a spi_read, which will overwrite the initial result.

It looks as if the extra spi_read was accidentally introduced in commit
a5e7363c ("staging:iio:gyro:adis16130 drop control of adc resolution."), quite
likely due to a bad rebase.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>