[SCSI] bfa: fix the issue of not handling scsi_cmnd sg chaining case

Currently the driver doesn't take into consideraion of possible sg chaining
when it walks through the sg list. This is fixed by using the sg_next()
which automatically handles the chaining case. Obosolete code is removed
as a result of this change.

Signed-off-by: Jing Huang <huangj@brocade.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>