iio: scd4x: check return of scd4x_write_and_fetch

iio: scd4x: check return of scd4x_write_and_fetch

Clang static analysis reports this problem
scd4x.c:474:10: warning: The left operand of '==' is a
  garbage value
  if (val == 0xff) {
      ~~~ ^
val is only set from a successful call to scd4x_write_and_fetch()
So check it's return.

Fixes: 49d22b695cbb ("drivers: iio: chemical: Add support for Sensirion SCD4x CO2 sensor")
Signed-off-by: Tom Rix <trix@redhat.com>
Link: https://lore.kernel.org/r/20220301025223.223223-1-trix@redhat.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>