power: smb5: Obtain DC max voltage from WLS PSY

The default DC max voltage in the absence of a WLS PSY is 12 V.
If WLS PSY is present, obtain this value from it.

Change-Id: If41b747846937b5e4f5f7755753c5827741d5dbb
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>