leds: qti-flash: Support zero duration for flash safety timer

The minimum timeout that can be set is 10 ms due to the clamping being
done in led_set_flash_timeout() in the flash LED class framework. Change
the minimum to allow zero timeout to be set, and also disallow setting
strobe in this case.

Change-Id: Ic34057197c05daf640c2a7d21cbae8e4638319ee
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>