[SCSI] qla4xxx: add async scan support

qla4xxx has the old school startup/probe where it finds presetup sessions
in its flash and then attempts to log into them before returning from the
probe. This however, makes it very simple to add a iscsi class scan finished
helper which the driver can use.

In future patches Dave or I will rip apart the driver to make it more
like qla2xxx, but for now this is a very simple two line patch which
fixes the problem of trying to figure out when the initial sessions
are done being scanned.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Cc: David Somayajulu <david.somayajulu@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>