mfd: omap-usb-tll: Fix error message

omap_enable/disable_tll() can fail if TLL device is not
initialized. It could be due to multiple reasons and not only
due to missing platform data.

Also make local variables static and use 'struct device *'
instead of 'struct platform_device *' for global reference.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>