leds: qti-flash: Enable zero brightness symmetry distribution

Currently, when a single channel's brightness is set, and then cleared
to zero subsequently, the LED stays on instead of getting turned off
because symmetrical brightness distribution logic does not allow the
paired channel's brightness to be set to zero. Fix this.

It was also observed that the brightness of a channel that is getting
turned off doesn't show up as zero, so fix that as well.

In addition, strengthen error-checking of the "qcom,led-mask" device
tree property.

Change-Id: I245ec2177b2ebe0f567eb96397dd13a05a587d60
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>