sc16is7xx: remove useless "uart_handle_cts_change"

SC16is7xx has feature for auto hardware flow control using RTS/CTS,
so we don't need "uart_handle_cts_change" to invoke "start_tx/stop_tx"
for flow control.

In addition, for software CTS, interrupt "SC16IS7XX_IIR_CTSRTS_SRC"
just report the nCTS change of state from active(low) to inactive(high),
driver don't know when the state of nCTS change from incactive to active,
so can't resume the transmission when a peer device is again ready and
re-assert nRTS signal.

Signed-off-by: Wills Wang <wills.wang@live.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>