commit d1b12075 upstream.

Calling pwm_config() with a period equal to zero always results in
error (-EINVAL) and pwm chip config method is never called.
Signed-off-by: Olivier Sobrie <olivier@sobrie.be>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>