Do not print misleading completion messages on trim failure

I/fstrim  (  124): Invoking FITRIM ioctl on /rom
E/fstrim  (  124): FITRIM ioctl failed on /rom
I/fstrim  (  124): Trimmed 18446744073709551615 bytes on /rom

Yeah, right!

Change-Id: I9996d15ced97da0ade576dfc9a059b2591504af1