ssa.c: Cosmetic change to variable name in ssa_downstream_handler

For improved code clarity, use i rather than slot for index into fd_to_conn
structure. This was pointed out in previous commit
7d9fd997a3576df85aa4f50e6ed57b0f920901d4
ssa.c: In ssa_check_listen_events, fix fd index of downstream connection tha
t is being closed

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