Missing critical phys_to_virt in lib/swiotlb.c

Adds missing call to phys_to_virt() in the
lib/swiotlb.c:swiotlb_sync_sg() function.  Without this change, a kernel
panic will always occur whenever a SWIOTLB bounce buffer from a
scatter-gather list gets synced.

Signed-off-by: David Moore <dcm@acm.org>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>