xhci: dbc: Don't use generic xhci erst allocation and free functions

The generic erst allocation and free functions take struct xhci_hcd pointer
as a parameter. Create own erst helpers for DbC in order to decouple xhci
and DbC

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20200723144530.9992-5-mathias.nyman@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>