leds: leds-qpnp: Utilize SMBB Boost Regulator for flash LED

Enable SMBB boost regulator when flash LED is turned on in flash
mode and disable it when flash LED is turned off. This avoids
losing USB when flash is turned on.

Change-Id: I31f22dc0f127bd4765466621e3c0882e15838d22
Signed-off-by: Chun Zhang <chunz@codeaurora.org>