USB: ehci-omap: Fix autoloading of module

The module alias should be "ehci-omap" and not
"omap-ehci" to match the platform device name.
The omap-ehci module should now autoload correctly.

CC: stable@vger.kernel.org

Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>