Fix reset glitch in pwmout_write()

Resetting the timer every time the duty cycle is changed causes unwanted glitches to be produced on the output. These glitches manifest as a tone at the update frequency when PWM is used for class D style audio.
Fix reset glitch in pwmout_write() on LPC13XX

Fixed the same problem as https://github.com/mbedmicro/mbed/pull/289 on the LPC13XX.
Merge pull request #289 from neilt6/master

LPC11UXX and LPCU13XX - Fix reset glitch in pwmout_write()