iio: meter: ade7759: add error handling in _reset and _stop_device

This patch adds the error handling for the value returned from
ade7759_spi_read_reg_16. With this patch, the following randconfig
warnings get fixed automatically.

drivers/staging/iio/meter/ade7759.c:224:6: warning: ‘val’ may be
used uninitialized in this function [-Wmaybe-uninitialized]
drivers/staging/iio/meter/ade7759.c:309:6: warning: ‘val’ may be
used uninitialized in this function [-Wmaybe-uninitialized]

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>