UBI: cleanup LEB start calculations

Wrong macro was used in calculating the data offset: UBI_EC_HDR_SIZE instead of
UBI_VID_HDR_SIZE. The data offset should be VID header offset + VID header size
(aligned to the minimum I/O unit).

This was not a bug only because currently UBI_EC_HDR_SIZE and UBI_VID_HDR_SIZE
have the same value of 64 bytes.

Commit message was amended by Artem.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>