close enable_fd when stopping PWM output (#197)

Issue #197: JesseMcL raised the issue that running
PWM.start() and PWM.start() in a loop will eventually
exhaust the number of open file descriptors and
PWM.start() will raise the error:

  RuntimeError: problem with sysfs file.

The file descriptor for the enable file should
be closed in pwm_disable() to avoid the leak

Signed-off-by: Drew Fustini <drew@pdp7.com>

Author: pdp7

source/c_pwm.c

@@ -717,6 +717,9 @@ BBIO_err pwm_disable(const char *key)
 #endif
 
             //close the fd
+#ifdef BBBVERSION41
+            close(pwm->enable_fd);
+#endif
             close(pwm->period_fd);
             close(pwm->duty_fd);
             close(pwm->polarity_fd);