leds: qpnp-flash-v2: fix a possible NULL pointer access

qpnp_flash_led_prepare() is a global function used by clients
like camera flash driver. Add a check to validate the trigger
pointer before using it to obtain LED class device.

Change-Id: I72c0690c13629cb3dd2ef6b9fd084835e58aa410
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>