dbc_dma_alloc_coherent() takes struct xhci_hcd pointer as an parameter, but
does nothing more than calls dma_alloc_coherent().
Remove it and call dma_alloc_coherent() directly instead.

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/20200723144530.9992-6-mathias.nyman@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>