xhci: dbc: Use dbc structure in the request completion instead of xhci_hcd

The dbc request completion callback doesn't need a xhci_hcd pointer.
The only user of the xhci_hcd pointer in dbgtty request callback was
the xhci_warn() function. Change it to dev_warn() instead.

While changing the callback function parameter to dbc in struct
xhci_requeset, move the struct xhci_request declaraion down a bit in the
header file to avoid compiler warinings

No functional changes
This change helps decoupling xhci and DbC

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20200723144530.9992-18-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>