gpio: aspeed: Don't attempt to debounce if disabled

We warn the user at driver probe time that debouncing is disabled.
However, if they request debouncing later on we print a confusing error
message:

 gpio_aspeed 1e780000.gpio: Failed to convert 5000us to cycles at 0Hz: -524

Instead bail out when the clock is not present.

Fixes: 5ae4cb94 (gpio: aspeed: Add debounce support)
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-aspeed.c

@@ -646,6 +646,9 @@ static int enable_debounce(struct gpio_chip *chip, unsigned int offset,
 	int rc;
 	int i;
 
+	if (!gpio->clk)
+		return -EINVAL;
+
 	rc = usecs_to_cycles(gpio, usecs, &requested_cycles);
 	if (rc < 0) {
 		dev_warn(chip->parent, "Failed to convert %luus to cycles at %luHz: %d\n",