drivers/char: allow using both dbgp=xhci and dbgp=ehci

drivers/char: allow using both dbgp=xhci and dbgp=ehci

This allows configuring EHCI and XHCI consoles separately,
simultaneously, such that e.g. one can be used for the console and the
other by the debugger.

This changes string_param() to custom_param() in both ehci and xhci
drivers. Both drivers parse only values applicable to them.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Jan Beulich <jbeulich@suse.com>