[SCSI] ibmvscsi: prevent scsi commands being sent in invalid state

There is a window where we can be re-enabling an adapter, but
still allow SCSI commands to be sent to the target.  This fix
sets our window (request_limit) to -1 as soon as we know the
adapter is being reenabled, and closes a very teeny tiny
window where we could set the window back to 1 before we
grab a lock.

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