leds: qpnp-flash-v2: Fix a missing return type for a function

qpnp_flash_led_thermal_config() was missing a return type triggering
a compiler warning.

Change-Id: Ic0dedbad17b3069eb1750999e886817ff00c9af2
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>