NTB: remove redundant setting of DB valid mask

db_valid_mask is set at two places, once within
amd_init_ntb(), and again within amd_init_dev().
Since amd_init_ntb() is actually called from
amd_init_dev(), setting db_valid_mask from
former does not really make sense. So remove it.

Signed-off-by: Arindam Nath <arindam.nath@amd.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>