leds: qpnp-flash-v2: add LED_KEEP_TRIGGER flag to switch devices

LED_KEEP_TRIGGER flag needs to be set on switch devices so that
the trigger will not be removed when the brightness is set to 0.
Without this, when an user sets brightness to 0 on switch device
and try to use camera flash, qpnp_flash_led_prepare() will fail
which in turn fails the camera flash operation.

Change-Id: I521bc6c6750bafc742dccac90ef585bc250b0270
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>