usb: ulpi: Define a *otg_ulpi_create no-op

Building a kernel for imx_v4_v5_defconfig with CONFIG_USB_ULPI disabled, results
in the following error:

arch/arm/mach-imx/built-in.o: In function 'pca100_init':
platform-mx2-emma.c:(.init.text+0x6788): undefined reference to 'otg_ulpi_create'
platform-mx2-emma.c:(.init.text+0x682c): undefined reference to 'mxc_ulpi_access_ops'

Fix this by providing a no-op definition of *otg_ulpi_create for the case when
CONFIG_USB_ULPI is not defined.

Acked-by: Igor Grinberg <[e-mail]>
Signed-off-by: Fabio Estevam <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

include/linux/usb/ulpi.h

···
 
 /*-------------------------------------------------------------------------*/
 
+#if IS_ENABLED(CONFIG_USB_ULPI)
 struct usb_phy *otg_ulpi_create(struct usb_phy_io_ops *ops,
 					unsigned int flags);
+#else
+static inline struct usb_phy *otg_ulpi_create(struct usb_phy_io_ops *ops,
+					      unsigned int flags)
+{
+	return NULL;
+}
+#endif
 
 #ifdef CONFIG_USB_ULPI_VIEWPORT
 /* access ops for controllers with a viewport register */