ARM: omap3_logic: Fix SPL boot failure when EHCI enabled

Some of the USB code is still being built into SPL even when the
SPL menu options have it explicitly disabled for SPL. Unit there is
a better solution, This patch undefines CONFIG_USB_EHCI_OMAP when
building SPL which reduces the code and lets the board boot again.

Fixes: 25e4ff45 ("ARM: omap3_logic: Enable OMAP EHCI support
for SOM-LV Boards")

Signed-off-by: Adam Ford <aford173@gmail.com>