leds: leds-qpnp-flash: detect open circuit fault in LED usage

There is chance hardware may come with bugs that LEDs are not
installed. When it happens, usage of flash LED may cause hardware
damage. Therefore, it is essential to detect open circuit fault so
that subsequent usage may decide whether to enable flash LED or not.

Change-Id: I69d8119902ae0d2eb587df6cfc04cb6a56f59118
Signed-off-by: Chun Zhang <chunz@codeaurora.org>