leds: leds-qpnp: bug fix for NULL pointer dereference in qpnp_xxx_set()

In previous merged commit: e7bca9b9, there is a bug about NULL
pointer dereference in qpnp_xxx_set functions. When led_mode
is MANUAL_MODE, there will be no PWM config data, so "pwm_cfg"
is a NULL pointer in this situation. Fix this by using it only
when the PWM config data exists.

CRs-Fixed: 622683
Change-Id: Ife218cc352e9620e86e0dda96335b6a43bdb65fb
Signed-off-by: Xu Kai <kaixu@codeaurora.org>