leds: qpnp-flash-v2: add delay between LMH mitigation and LED strobe

As per the hardware recommendation, a delay of 500 us is needed
after LMH mitigation is enabled by software and before the flash
LED is strobed. Add it.

Change-Id: Ib70efbb7267510ffb9d95c4aba8566bf6e777c0f
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>