NTB: Clear property bits in BAR value

The lower bits read from a BAR register will contain property bits
that we do not care about. Clear those so that we can use the BAR
values for limit and xlat registers.

Reported-by: Conrad Meyer <cem@freebsd.org>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>