[SCSI] qla2xxx: use kthread_ API

Use the kthread_ API instead of opencoding lots of hairy code for kernel
thread creation and teardown.

Also switch from semaphore-based thread wakeup to wake_up_process.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-By: Andrew Vasquez <andrew.vasquez@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>