iommu/arm-smmu: Delete an unnecessary check before free_io_pgtable_ops()

The free_io_pgtable_ops() function tests whether its argument is NULL
and then returns immediately. Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Will Deacon <will.deacon@arm.com>