iio: at91_adc: Use devm_kcalloc to allocate arrays

Use dev_kcalloc instead of devm_kzalloc to allocate arrays since it is
semantically more appropriate.

While we are at it the patch also fixes the following coccinelle warning:
	drivers/iio/adc/at91_adc.c:277:25-31: ERROR: application of sizeof to pointer

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-By: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>