usb: xhci: make function xhci_dbc_free_req static

Function xhci_dbc_free_req is local to the source and does not need to
be in global scope, so make it static.

Cleans up sparse warning:
symbol 'xhci_dbc_free_req' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Cc: Mathias Nyman <mathias.nyman@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>