serial: 8250: extract serial8250_clear_interrupts()

serial: 8250: extract serial8250_clear_interrupts()

On three places in 8250_port.c, the interrupts are cleared by reading 4
registers. Extract this to a separate function:
serial8250_clear_interrupts(). And call it from all the places.

Note autoconfig_irq() now uses serial_port_in() instead of serial_in().
But they are the same, in fact (modulo parameter).

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20250611100319.186924-18-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>