power: smb135x-charger: update USB online handling logic

Charger driver updates USB 'online' property to indicate charging
state. Update this logic to set online to true/false on USB
insertion/removal if charging is enabled by the user.

CRs-Fixed: 899239
Change-Id: I6e0b3b0ea0a2fcb378c0eab68f0ab5e195c49926
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>