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

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

Fixes the following W=1 warning:

 drivers/pwm/pwm-bcm-iproc.c:147:12: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

Cc: Ray Jui <rjui@broadcom.com>
Cc: Scott Branden <sbranden@broadcom.com>
Cc: Yendapally Reddy Dhananjaya Reddy <yendapally.reddy@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>

Author: Lee Jones

drivers/pwm/pwm-bcm-iproc.c

@@ -143,8 +143,7 @@ static int iproc_pwmc_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 		value = rate * state->duty_cycle;
 		duty = div64_u64(value, div);
 
-		if (period < IPROC_PWM_PERIOD_MIN ||
-		    duty < IPROC_PWM_DUTY_CYCLE_MIN)
+		if (period < IPROC_PWM_PERIOD_MIN)
 			return -EINVAL;
 
 		if (period <= IPROC_PWM_PERIOD_MAX &&