NTB: amd: Silence shift wrapping warning in amd_ntb_db_vector_mask()

This code triggers a Smatch warning:

    drivers/ntb/hw/amd/ntb_hw_amd.c:336 amd_ntb_db_vector_mask()
    warn: should '(1 << db_vector)' be a 64 bit type?

I don't think "db_vector" can be higher than 16 so this doesn't affect
runtime, but it's nice to silence the static checker warning and we
might increase "ndev->db_count" in the future.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>