[SCSI] qla2xxx: Generalize srb structure usage.

Lay groundwork for adding alternative asynchronous operations by
generalize and extending the SRB structure.  This allows for
follow-on patches to add support for:

- Asynchronous logins.
- ELS/CT passthru requests.
- Loopback requests.
- Non-blocking mailbox commands (ABTS, Task Management, etc).

Signed-off-by: Andrew Vasquez <andrew.vasquez@qlogic.com>
Signed-off-by: Giridhar Malavali <giridhar.malavali@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>