iio: adc: twl4030-madc: Remove set but unused variables 'len'

iio: adc: twl4030-madc: Remove set but unused variables 'len'

Fixes the following W=1 kernel build warning(s):

 drivers/iio/adc/twl4030-madc.c: In function ‘twl4030_madc_threaded_irq_handler’:
 drivers/iio/adc/twl4030-madc.c:475:9: warning: variable ‘len’ set but not used [-Wunused-but-set-variable]
 475 | int i, len, ret;
 | ^~~

Cc: Sebastian Reichel <sre@kernel.org>
Cc: J Keerthy <j-keerthy@ti.com>
Cc: Mikko Ylinen <mikko.k.ylinen@nokia.com>
Cc: Amit Kucheria <amit.kucheria@canonical.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>