staging: iio: adc: mxs-lradc: Change type in printf format string

Wrong type in printf format string, requires 'int'
but the argument type is 'unsigned int'

This was found using a static code analysis program called cppcheck

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>