iio: at91: fix dev var name in at91_adc_trigger_handler

/opt/work/linux-2.6/drivers/iio/adc/at91_adc.c: In function 'at91_adc_trigger_handler':
/opt/work/linux-2.6/drivers/iio/adc/at91_adc.c:83:22: error: 'indio_dev' undeclared (first use in this function)
/opt/work/linux-2.6/drivers/iio/adc/at91_adc.c:83:22: note: each undeclared identifier is reported only once for each function it appears in

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>