tty: serial: imx: Fix broken RS485

tty: serial: imx: Fix broken RS485

When about to transmit the function imx_uart_start_tx is called and in
some RS485 configurations this function will call imx_uart_stop_rx. The
problem is that imx_uart_stop_rx will enable loopback in order to
release the RS485 bus, but when loopback is enabled transmitted data
will just be looped to RX.

This patch fixes the above problem by not enabling loopback when about
to transmit.

This driver now works well when used for RS485 half duplex master
configurations.

Fixes: 79d0224f6bf2 ("tty: serial: imx: Handle RS485 DE signal active high")
Cc: stable <stable@kernel.org>
Signed-off-by: Rickard x Andersson <rickaran@axis.com>
Tested-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Link: https://lore.kernel.org/r/20240221115304.509811-1-rickaran@axis.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>