usb: chipidea: otg: set host_request_flag for gadget

Set host_request_flag if the current peripheral wants to take host role
via changing a_bus_req or b_bus_req by user application.

Acked-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Li Jun <jun.li@nxp.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>