usb: ehci: Split out struct ehci_ctrl definition

Move the struct ehci_ctrl defition from ehci-hcd.c into ehci.h
so it can be re-used by drivers. In particular, the mv_udc driver
can benefit from this move.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Lei Wen <leiwen@marvell.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Stefano Babic <sbabic@denx.de>