st: clear ILI if Medium Error

Some drives set the ILI flag together with MEDIUM ERROR sense code.
Clear the ILI flag in this case so that the medium error will be
handled.  The problem was reported by Maurizio Lombardi.

Signed-off-by: Kai Mäkisara <kai.makisara@kolumbus.fi>
Reviewed-by: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>