pwm: bcm-kona: Remove impossible comparison when validating duty cycle

pwm: bcm-kona: Remove impossible comparison when validating duty cycle

'dc' here is an unsigned long, thus checking for <0 will always
evaluate to false.

Fixes the following W=1 warning:

 drivers/pwm/pwm-bcm-kona.c:141:35: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Ray Jui <rjui@broadcom.com>
Cc: Scott Branden <sbranden@broadcom.com>
Cc: bcm-kernel-feedback-list@broadcom.com
Cc: linux-pwm@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>