samples: fade_led: fix build error when PWM is not enabled

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
a5fd92b0 ("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>