As the limitation of RR312x's dma engine, the HBA can not access host memory
over 12GB.  This fixes

https://bugzilla.kernel.org/show_bug.cgi?id=14311

[alan: resurrected bug from 2009 and pushed upstream]
Reported-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: HighPoint Linux Team <linux@highpoint-tech.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>