staging:iio:lpc32xx_adc: Use resource_size instead of opencoding it

Fixes the following error from coccicheck:
	drivers/staging/iio/adc/lpc32xx_adc.c:153:43-46: ERROR: Missing resource_size with res

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Roland Stigge <stigge@antcom.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>