serial: stm32: fix the conditional expression writing

In the function stm32_usart_init_port, intent of the code maybe when
irq returns a value of zero, the return should be '-ENODEV'. But the
conditional expression '? :' maybe clerical error, it should be
'?:' to make '-ENODEV' work.
But in fact, as the example in platform.c is
  * int irq = platform_get_irq(pdev, 0);
  * if (irq < 0)
  * return irq;
So the return value of zero is unnecessary to check, at last remove
the unnecessary '?: -ENODEV'.

Co-developed-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Link: https://lore.kernel.org/r/20210811105136.25392-1-tangbin@cmss.chinamobile.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>