iommu/vt-d: Check return value of acpi_bus_get_device()

iommu/vt-d: Check return value of acpi_bus_get_device()

commit c0df975f9045d6b2b13d88746e628ac308ff49ea upstream.

Checking adev == NULL is not sufficient as
acpi_bus_get_device() might not touch the value of this
parameter in an error case, so check the return value
directly.

Fixes: ed40356b5fcf1ce28e026ab39c5b2b6939068b50
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>