xhci: dbc: Don't use generic xhci context allocation for dbc

xhci: dbc: Don't use generic xhci context allocation for dbc

The DbC context is different from the xhci device context.
It's a lot smaller as it only contains three 64 bytes sub-contexts;
the info, endpoint-out, and endpoint-in contexts. In total 192 bytes.
The context size (CSZ) field in HCCPARAMS1 xhci register does not alter
DbC context size like it does for xhci device contexts.

So don't use the geneic xhci context memory allocation, or the
dma pool that is intended for xhci device contexts.

In addition to saving memory this also helps decoupleing xhci and dbc code.

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20200723144530.9992-19-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>