serial: imx.c: fix CTS trigger level lower to avoid lost chars

The imx CTS trigger level is left at its reset value that is 32
chars. Since the RX FIFO has 32 entries, when CTS is raised, the
FIFO already is full. However, some serial port devices first empty
their TX FIFO before stopping when CTS is raised, resulting in lost
chars.

This patch sets the trigger level lower so that other chars arrive
after CTS is raised, there is still room for 16 of them.

Signed-off-by: Valentin <Longchamp&lt;valentin.longchamp@epfl.ch>
Tested-by: Philippe <Rétornaz&lt;philippe.retornaz@epfl.ch>
Acked-by: Wolfram <Sang&lt;w.sang@pengutronix.de>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>