HW flow control should be enable/disable by userspace.
It can be set by tty option: options.c_cflag of CRTSCTS
bit.
This bug is found by testing UART BT AR3001 chip,
this chip will wake up the chip by set RTS to high,
it needs frist disable HW flow control, set RTS,
then disable HW flow control.
Signed-off-by: Zhang Jiejing <jiejing.zhang@freescale.com>