[SCSI] qla2xxx: Cache swl during fabric discovery.

Rather than continuously allocating and freeing swl within the discovery
process, simply pre-allocate it the first time that it's needed, cache it
through the rest of the lifecycle of the driver and free it at module unload.

Signed-off-by: Andrew Vasquez <andrew.vasquez@qlogic.com>
Signed-off-by: Chad Dupuis <chad.dupuis@qlogic.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>