cbus-tahvo-usb: Fix tahvo_usb_device pointer

The static tahvo_usb_device is uninitialized, but used
in the otg interrupt handler. This results in a NULL pointer
dereference on interrupt.
Fix this by storing a struct tahvo_usb pointer instead of
a platform device.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>