usb: fix build error without CONFIG_USB_PHY

on i386:

drivers/built-in.o: In function `ci_hdrc_probe':
core.c:(.text+0x20446b): undefined reference to `of_usb_get_phy_mode'

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>