CLEANUP: stconn: use a single function to know if SC may send to SE

CLEANUP: stconn: use a single function to know if SC may send to SE

sc_is_send_allowed() is now used everywhere instead of the combination
of cs_tx_endp_ready() && !cs_tx_blocked(). There's no place where we
need them individually thus it's simpler. The test was placed in cs_util
as we'll complete it later.