usb: mv_udc: Make use of struct ehci_ctrl

The usb_lowlevel_init() call already fills and passes back struct
ehci_ctrl , which readily contains correctly determined address of
the port register block address computed from values from controller
configuration registers. Leverage this and make use of this value
as this makes the code mode universal, but also gets us rid of the
CONFIG_USB_REG_BASE configuration option.

Moreover, this patch cleans up the usb_gadget_register_driver() call
a little by correcting the error handling. Note the usb_lowlevel_init()
and mvudc_probe() are now called in reversed order, but this has no
impact on the code.

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>