serial: stm32: fix a recursive locking in stm32_config_rs485

Remove spin_lock_irqsave in stm32_config_rs485, it cause recursive locking.
Already locked in uart_set_rs485_config.

Fixes: 1bcda09d ("serial: stm32: add support for RS485 hardware control mode")
Signed-off-by: Borut Seljak <borut.seljak@t-2.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>