iio: mxs-lradc: Check the return value from stmp_reset_block()

stmp_reset_block() may fail, so let's check its return value and propagate it in
the case of error.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>