leds: qpnp-flash-v2: Do not clear led_on when flash/torch brightness is 0

Currently, led_on flag is cleared whenever the brightness is set
to 0. This is incorrect as the led_on flag should be cleared only
when switch device is disabled. Fix it.

Change-Id: I7d70cb9588b55610dbc8eaf57338490ce82a0917
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>