xhci: dbc: allow setting device serial number through sysfs

xhci: dbc: allow setting device serial number through sysfs

Add dbc_serial sysfs attribute to allow changing the serial number
string descriptor presented by the debug device when a host requests a
string descriptor with iSerialNumber index.

Value can only be changed while debug capability (DbC) is in disabled
state to prevent USB device descriptor change while connected to a USB
host.

The default value is "0001".
The field length can be from 1 to 126 characters.
String is terminated at null or newline, driver does not support empty
string.

[ mn: Improve commit message and sysfs entry documentation ]

Signed-off-by: Łukasz Bartosik <ukaszb@chromium.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://patch.msgid.link/20260120181148.128712-3-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>