settings: file: Fix possible success status while failed to write
settings: file: Fix possible success status while failed to write

Assuming that fs_seek has been successful; in case when fs_write
would be unsuccessful and fs_close, that follows, would be successful,
the success code would have been returned for the entire procedure,
although it has failed.

Signed-off-by: Dominik Ermel <1a1d45a9cc0c98a37f8d0a0d2dbe3cacc0b2344f@nordicsemi.no>

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