[SCSI] qla2xxx: Honour "Extended BB credits" bit for CNAs.

We now enable/disable "Additional Receive Credits" in f/w based on nvram
parameter "Extended_BB_Credits" bit (i.e. Enhanced-Features: at offset 0x196).
This is applicable only for GEN2 CNAs.

Signed-off-by: Santosh Vernekar <santosh.vernekar@qlogic.com>
Signed-off-by: Giridhar Malavali <giridhar.malavali@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>