Some boards may define LEDs but disable them by default due to conflicts
with other peripherals. In this case, the new sample code introduced in
a5fd92b ("samples: fade_led: use all available LEDs") fails to build
with a missing symbol error.
Fix this by checking if the PWM device (the parent of the LED alias) is
enabled before using it. This implicitly discards invalid aliases.
Signed-off-by: Luca Burelli <l.burelli@arduino.cc>
187a8ad09b