ARM: mx3/mx31_3ds: fix compiler warning without CONFIG_USB_ULPI

This fixes:

	arch/arm/mach-mx3/mach-mx31_3ds.c:249: warning: 'mx31_3ds_host2_init' defined but not used

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>