[DISCO_F407VG] Fixed PWM PA_3 (and PC_9) output issue.
Refer to:
http://mbed.org/users/mbed_official/code/mbed-src/issues/4
Merge pull request #261 from dinau/pwm_pa3_issue

[DISCO_F407VG] Fixed PWM PA_3 (and PC_9) output issue.