greybus: kernel_ver.h: provide pwm_is_enabled()

In the 4.7-rc1 kernel release, PWMF_ENABLED is removed and
pwm_is_enabled() is the correct way to test if a pwm device is enabled,
so provide a version of that function that will work on all older
kernels and change the pwm.c driver to use it so that it will work on
newer kernels as well.

Tested:
	Tree now builds successfully against 3.14.y, 4.4.y, 4.5.y,
	4.6.y, and 4.7-rc2 kernels

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