serial: stm32: Clear prev values before setting RTS delays

serial: stm32: Clear prev values before setting RTS delays

The code lacks clearing of previous DEAT/DEDT values. Thus, changing
values on the fly results in garbage delays tending towards the maximum
value as more and more bits are ORed together. (Leaving RS485 mode
would have cleared the old values though).

Fixes: 1bcda09d2910 ("serial: stm32: add support for RS485 hardware control mode")
Cc: stable <stable@kernel.org>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220627150753.34510-1-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>