leds: qti-flash: Add individual APIs to enable and disable switch

Currently qti_flash_led_switch_brightness_set() is used to enable or
disable switch. Split it into separate APIs to enable and disable
switch. This will make it easier to control switches which is required
for implementing slow-sync flash feature.

Change-Id: I668ab872d1d646f97ed8fbfba4dfca6b4db5bc70
Signed-off-by: Shyam Kumar Thella <sthella@codeaurora.org>