drivers: uhc: rework transfer buffer handling

The current approach is a bit impractical in the upper layer.
This patch removes the two fifos that hold the transfer buffers
and replaces them with a byte array for the setup packet and
a pointer to a data buffer. The data buffer is mandatory for
all types of transfers except control without a data stage.
The waste of eight unused bytes for non-control transfers should
be insignificant, since an additional pointer would be at least
half of it, and then there would be the overhead of handling it.

This patch also clean up the transfer flags, rename owner to callback
as it reflects the upper layer use case, and add an additional member
to hold the pointer to the USB device (peripheral on the bus).

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>