usb: host: ehci-platform: BUG_ON() to WARN_ON() on probe

usb_ehci_pdata is certainly required in ehci-platform driver.
This patch avoids using BUG_ON() from driver,
and return from probe with WARN_ON() if pdata was NULL.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>