power: smb5: Simplify voltage estimation in the case of HVDCP2

The MID_CHG channel returns unreliable values above 10.5V, which can
only be 12V for HVDCP2, therefore simply return 12V instead of reading
status bits from a register.

Also correct the debug logging level of the information in the Source
Change interrupt handler to the right level.

Change-Id: I32ab068edbc6fcddd9f4acc8562596cb895388fa
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>