UBIFS: fix ubifs_compress commentary

Update the comment for ubifs_compress(), which incorrectly states that it
returnsa success/failure indicator.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>