iio: chemical: sunrise_co2: set val parameter only on success

iio: chemical: sunrise_co2: set val parameter only on success

Clang static analysis reports this representative warning

sunrise_co2.c:410:9: warning: Assigned value is garbage or undefined
  *val = value;
       ^ ~~~~~

The ealier call to sunrise_read_word can fail without setting
value.  So defer setting val until we know the read was successful.

Fixes: 4eab0ad66949 ("iio: chemical: Add Senseair Sunrise 006-0-007 driver")
Signed-off-by: Tom Rix <trix@redhat.com>
Link: https://lore.kernel.org/r/20211224150833.3278236-1-trix@redhat.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>