db: skip column type change if already set

This patch fix a failure in fn_db_change_column_type if the target type
was already applied manually on the column.
In this case the function will now succeed instead of failing and
causing the upgrade process to exit

Change-Id: Ifa88af621f21894d6d575a1e44f15a29d75b9c87
Signed-off-by: emesika <emesika@redhat.com>