i2c: tiny-usb: do not use stack as URB transfer_buffer

Patch fixes i2c-tiny-usb not to use stack as URB transfer_buffer. URB buffers
need to be DMA-able, which stack is not.

Patch is only compile tested.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>