usb: musb: remove duplicated port mode enum

include/linux/usb/musb.h already defines enum for musb port mode, so
remove the duplicate in musb_core.h and use the definition in musb.h.

Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>