iio: adc: mt6577_auxdac: use devm_platform_ioremap_resource

Reduces boilerplate.  Identified by coccinelle

CHECK   drivers/iio/adc/mt6577_auxadc.c
drivers/iio/adc/mt6577_auxadc.c:257:1-18: WARNING: Use devm_platform_ioremap_resource for adc_dev -> reg_base

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Zhiyong Tao <zhiyong.tao@mediatek.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>