sd-device: ignore error in device_cache_sysattr_value() and propagate original error...

sd-device: ignore error in device_cache_sysattr_value() and propagate original error code

There are three calls of device_cache_sysattr_value(). Two of them are
just caching the value. Hence, let's ignore its failure, and propagate
original error code.

One exception is the last call in sd_device_get_sysattr_value().
Unfortunately, it returns `const char *` instead of `char *`. So,
sd_device object must have the reference of the returned value.
Hence, error in updating the cache by device_cache_sysattr_value()
is critical, and we need to propagate the error in that case.