serial: 8250: Use cached port name directly in messages

Since we have port name stored in struct uart_port, we better to use
that one instead of open coding.

This will make it one place source for easier maintenance or
modifications.

While here, replace printk_ratelimited(KERN_INFO ) by pr_info_ratelimited().
It seems last printk() call in 8250_port.c.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>