serial: qcom-geni: Fix RTS behavior with flow control

serial: qcom-geni: Fix RTS behavior with flow control

When userspace enables flow control (CRTSCTS), the driver
deasserts RTS even when the receive buffer has space. This prevents the
peer device from transmitting, causing communication to stall.

The root cause is that the driver unconditionally uses manual RTS control
regardless of flow control mode. When CRTSCTS is set, the hardware should
automatically manage RTS based on buffer status, but the driver overrides
this by setting manual control.

Fix this by introducing port->manual_flow flag. In set_termios(), disable
manual flow when CRTSCTS is set. In set_mctrl(), only assert
SE_UART_MANUAL_RFR when manual_flow is active. Verified by enabling and
disabling hardware flow control with stty.

Signed-off-by: Anup Kulkarni <anup.kulkarni@oss.qualcomm.com>
Link: https://patch.msgid.link/20260310104155.339010-1-anup.kulkarni@oss.qualcomm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>