UBI: fix signed-unsigned multiplication

There is signed multiplication assigned to unsigned ei.addr in io.c.
This causes wrong addresses for big multiplication.This patch solves the
problem.

Signed-off-by: Brijesh Singh <brijesh.s.singh@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>