iio: xilinx-xadc: Remove unused variable

'offset’ is not used in the function. Remove it.

Signed-off-by: Sachin Kamat <sachin.kamat@samsung.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>