usb: pd: Don't suspend charging unless changing voltages

To optimize charging during power negotiation and the voltage
is not changing, for example when transitioning from implicit
contract to the default 5V PDO, there is no requirement to
suspend the charging and hence setting the charger's
PROP_PD_CURRENT_MAX can be avoided. It is only needed when
changing voltages, in which case current limit should be
calculated based on pSnkStdby (2.5W), or when staying at the
same voltage and decreasing current.

Also fix the incorrect setting of PROP_CURRENT_MAX to
PROP_PD_CURRENT_MAX when doing a sink->source swap.

Change-Id: Ib53902459646e590df4dc7fcb00f833d5e8f41ed
Signed-off-by: Jack Pham <jackp@codeaurora.org>