serial: sc16is7xx: Fix IRQ number check behavior

The logical meaning of the previous version is wrong due to a typo.

If the IRQ equals 0, no interrupt pin is available and polling mode
shall be used.

Additionally, this fix adds a check for IRQ < 0 to increase robustness,
because documentation still says that negative IRQ values cannot be
absolutely ruled-out.

Fixes: 104c1b9dde9d ("serial: sc16is7xx: Add polling mode if no IRQ pin is available")
Signed-off-by: Andre Werner <[e-mail]>
Reviewed-by: Jiri Slaby <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Reviewed-by: Maarten Brock <[e-mail]>
Reviewed-by: Hugo Villeneuve <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/sc16is7xx.c

···
 	/* Always ask for fixed clock rate from a property. */
 	device_property_read_u32(dev, "clock-frequency", &uartclk);
 
-	s->polling = !!irq;
+	s->polling = (irq <= 0);
 	if (s->polling)
 		dev_dbg(dev,
 			"No interrupt pin definition, falling back to polling mode\n");