leds: qpnp-flash-v2: Acquire the bms_psy handle at runtime

Un-registering the power_supply notifier in its callback leads
to a "sleeping function from a invalid context" issue. Also, there
is no need to register for a power_supply notifier for the bms_psy
handle. Acquire this handle runtime, this also fixes the
sleeping-context issue.

Change-Id: I7c98446d88be3d2231ab30b53eebc0660e66b016
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>