ntb: intel: fix return value for ndev_vec_mask()

ndev_vec_mask() should be returning u64 mask value instead of int.
Otherwise the mask value returned can be incorrect for larger
vectors.

Fixes: e26a5843 ("NTB: Split ntb_hw_intel and ntb_transport drivers")

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Tested-by: Lucas Van <lucas.van@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>