dmar: fix uninitialised 'ret' variable in dmar_parse_dev()

This was introduced by commit 1886e8a9
("x64, x2apic/intr-remap: code re-structuring, to be used by both DMA
and Interrupt remapping"). It was causing bogus results to be returned
from dmar_parse_dev() when the first unit with the INCLUDE_ALL flag was
processed.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>