core.c: Cosmetic formatting change

Signed-off-by: Hal Rosenstock <hal@mellanox.com>
core.c: Use FIRST_DOWNSTREAM_FD_SLOT define rather than hard coded constant

Signed-off-by: Hal Rosenstock <hal@mellanox.com>
ssa.c: Rename ssa_downstream_send_db_update_ready to ssa_send_db_update_ready

in preparation for core + access support which shares this routine

Signed-off-by: Hal Rosenstock <hal@mellanox.com>