amba: tegra-ahb: Fix device leak on SMMU enable

amba: tegra-ahb: Fix device leak on SMMU enable

Make sure to drop the reference taken to the AHB platform device when
looking up its driver data while enabling the SMMU.

Note that holding a reference to a device does not prevent its driver
data from going away.

Fixes: 89c788bab1f0 ("ARM: tegra: Add SMMU enabler in AHB")
Cc: stable@vger.kernel.org	# 3.5
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>