iio: temperature: ltc2983: fix u32 read into a unsigned long long

Currently the read of temp using of_property_read_u32_index is reading
a u32 value into a unsigned long long.  This relies on machine endianness
to work correctly, so fix this by reading a u32 value and setting temp
to this value.

Addresses-Coverity: ("Reliance on integer endianness")
Fixes: f110f318 ("iio: temperature: Add support for LTC2983")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>