leds: leds-qpnp: use the proper pwm period in LPG mode

Previously, when the leds were working in LPG mode, the pwm
period was always set to the minimum value supported by hardware.
That's unreasonable. The better way is to set the period to the
expected value.

CRs-Fixed: 655566
Change-Id: I30b17cbfe98639ec132484f314cb9b9234e74354
Signed-off-by: Xu Kai <kaixu@codeaurora.org>