ibmvfc: Fix endless PRLI loop in discovery

Fixes a problem seen where sending a PRLI to a target
resulted in it sending a LOGO. This caused the ibmvfc
driver to go back through discovery again, which caused
another PRLI attempt, which caused another LOGO. Fix this
behavior by ignoring LOGO if we haven't even logged into
the target yet.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>