Refactor xhci_dbc_init(), splitting it into logical
parts closer to the Linux device model.

- Create the fake dbc device, depends on xhci strucure
- Allocate a dbc structure, xhci agnostic
- Call xhci_dbc_tty_probe(), similar to actual probe.

Adjustments to xhci_dbc_exit and xhci_dbc_remove are also needed
as a result to the xhci_dbc_init() changes

Mostly non-functional changes, except for creating the dbc sysfs
entry earlier, together with the dbc structure.
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20220216095153.1303105-2-mathias.nyman@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>