greybus: pwm: Fix compilation with v4.3 kernel

pwm_is_enabled() wasn't enabled by v4.2-rc1, but it was based of
v4.2-rc1. It was actually first included in v4.3.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>