xhci: dbc: Pass dbc pointer to dbc start and stop functions.

xhci: dbc: Pass dbc pointer to dbc start and stop functions.

xhci_dbc_start() and xhci_dbc_stop() functions only used xhci_hcd pointer
to get the dbc pointer.
Pass the dbc pointer instead of the xhci_hcd pointer as a parameter

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/20200723144530.9992-22-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>