leds: qti-tri-led: Cancel set_brightness_work before setting breath

Currently, the driver uses brightness_set_blocking() which is called
asynchronously in the set_brightness_work of LED class device. It can
cause issues when multiple set_brightness() requests coming along with
breath_store() requests and if any of the set_brightness() requests
are blocked.

To overcome this, cancel set_brightness_work of LED class device
to make sure all the previous set_brightness() requests have been
served or cancelled before setting breath.

Change-Id: Ib9a8527867ab1adf2b034642f5efda72df30aea0
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>