[SCSI] ibmvscsi: handle re-enable firmware message

New versions of the Power5 firmware can send a "re-enable" message to
the virtual scsi adapter.  This fix makes us handle the message
correctly.  Without it, the driver goes catatonic and the system crashes
unpleasantly.

Signed-off-by: Dave Boutcher <sleddog@us.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>