Signed-off-by: Anton Blanchard <anton@samba.org>

We have cards that want over 2MB of PCI consistent memory.  The
IOMAP_MAX_ORDER limit is just to catch bad drivers early, so we can bump
this a bit.

We want some room to grow but our maximum get_free_pages allocation on
ppc64 is currently 16MB, so it doesnt make sense to go above that.