iommu/arm-smmu-v3: Make arm_smmu_install_ste_for_dev return void

arm_smmu_install_ste_for_dev cannot fail and always returns 0, however
the fact that it returns int means that callers end up implementing
redundant error handling code which complicates STE tracking and is
never executed.

This patch changes the return type of arm_smmu_install_ste_for_dev
to void, to make it explicit that it cannot fail.

Signed-off-by: Will Deacon <will.deacon@arm.com>