power: qpnp-charger: Fix null pointer dereference error

Fix to prevent null pointer dereference error in the led brightness
set function by an initial check to determine whether the
power_supply structure is defined or not.

CRs-Fixed: 2166164
Change-Id: Ifcd6e55aa78c4c4d4dc539ac6ffe67263d198b47
Signed-off-by: Umang Agrawal <uagrawal@codeaurora.org>