pwm: lpc32xx: Don't change PWM_ENABLE bit in lpc32xx_pwm_config

lpc32xx_pwm_config() is supposed to set duty_ns and period_ns,
it should not change PWM_ENABLE bit.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Tested-by: Roland Stigge <stigge@antcom.de>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>