usb: pd: Re-enable SuperSpeed host by default

This reverts commit 85cc38fc ("usb: pd: Add support to set the
maximum speed via extcon") which unnecessarily restricted starting
USB host mode to high-speed only. Relax this so that USB host can
start in SuperSpeed when DFP mode is entered.

However, the only exception is if DisplayPort mode is discovered,
we need to drop down to high-speed only due to current hardware
limitations. This is handled during the VDM discovery stage.

Also use this opportunity to refactor all extcon_set_state_() calls
to helper functions that start/stop USB host/peripheral modes.

Change-Id: I2f32f70c57a8a6e2c34c8daf59dea931b445422f
Signed-off-by: Jack Pham <jackp@codeaurora.org>