ARM: am3517-evm: Disable CONFIG_USB_EHCI_OMAP in SPL

Found accidentally in omap3_logic, CONFIG_USB_EHCI_OMAP adds some
code size to SPL, so this patch disables it on the am3517-evm to
reduce the code a bit since it's tight for space.
Signed-off-by: NAdam Ford <aford173@gmail.com>

include/configs/am3517_evm.h

@@ -26,8 +26,11 @@
  * Enable CONFIG_USB_MUSB_HOST for Host functionalities MSC, keyboard
  * Enable CONFIG_USB_MUSB_GADGET for Device functionalities.
  */
-
+#ifdef CONFIG_SPL_BUILD
+#undef CONFIG_USB_EHCI_OMAP
+#else
 #define CONFIG_OMAP_EHCI_PHY1_RESET_GPIO	57
+#endif
 
 /* I2C */