usb: ehci-xilinx-usbps: Fix reboot issue in OTG mode

usb: ehci-xilinx-usbps: Fix reboot issue in OTG mode

This patch fix the reboot issue when OTG mode is used
and reset is performed without using OTG-host mode.

The reason is not initializaed ehci->caps/regs which
are initialized in ehci_xusbps_setup but this function
is called when host mode is used.
But uninitialization depends on these values.

Path: usb_hcd_platform_shutdown -> ehci_shutdown ->
ehci_silence_controller -> ehci_halt
where code expect filled ehci->regs value.

Change ehci behavior in OTG mode to add and remove hcd
which ensure that all structure are initialized
and ehci can be shutdown.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>