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: 5ae4cb94b3133 (gpio: aspeed: Add debounce support)
Signed-off-by: Joel Stanley <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

drivers/gpio/gpio-aspeed.c

···
 	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",