omap iommu: Fix Memory leak

The memory allocated for sgt structure is not freed on error
when sg_alloc_table is called in sgtable_alloc().

Signed-off-by: Satish Kumar <x0124230@ti.com>
Signed-off-by: Manjunatha GK <manjugk@ti.com>
Cc: Vimal Singh <vimal.newwork@gmail.com>
Signed-off-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>
Signed-off-by: Tony Lindgen <tony@atomide.com>