power: smbcharger: 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.

Change-Id: I09f793630c2ce98caa4a2c1b711bf7a4a9bd4eb5
Signed-off-by: Umang Agrawal <uagrawal@codeaurora.org>