musb_hdrc: host vbus and connect/disconnect fixes

Teach host side VBUS powerup and connect logic about the problem whereby
the HDRC core wrongly reports VBUS_ERROR during initial powerup.

Remove previous non-working workaround for tusb powering VBUS:  don't
try handling CPEN by hand.

Rest of the fix for disconnection issues in A_SUSPEND.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>