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>