leds: qti-flash: Limit off_time of switch to a maximum of 1280 ms

Currently, Flash LED peripheral supports a maximum safety timer expiry
of 1280 ms for flash mode where the target current can go as high as
1500 mA. If off_time is allowed more than that time period, then
flash LED can stay ON more than that safety timer period. Hence, clamp
off_time to not exceed 1280 ms.

Change-Id: Idbd8b41c5041c50e3f714be9efb0ad04b1ee9181
Signed-off-by: Shyam Kumar Thella <sthella@codeaurora.org>