usb: pd: Add extcon_blocking_sync in start_usb_host

Currently dp_notifier is getting called before usb
host mode is setup which is causing issue in setting
up usb_dp_combo mode due to which timeouts from dp
driver are seen.

This change blocks everything until usb host is
completely started ensuring usb_dp_combo mode is
successfully set, thus fixing the issue.

Change-Id: Iabddfbdf7f97edd3e4b655c833df246100892c2c
Signed-off-by: Rohith Kollalsi <rkollals@codeaurora.org>