usb: gadget: u_ether: add gether_opts for config caching

Currently, the net_device is allocated when the function instance is
created (e.g., in ncm_alloc_inst()). While this allows userspace to
configure the device early, it decouples the net_device lifecycle from
the actual USB connection state (bind/unbind). The goal is to defer
net_device creation to the bind callback to properly align the lifecycle
with its parent gadget device.

However, deferring net_device allocation would prevent userspace from
configuring parameters (like interface name or MAC address) before the
net_device exists.

Introduce a new structure, struct gether_opts, associated with the
usb_function_instance, to cache settings independently of the
net_device. These settings include the interface name pattern, MAC
addresses (device and host), queue multiplier, and address assignment
type.

New helper functions are added:
- gether_setup_opts_default(): Initializes struct gether_opts with
  defaults, including random MAC addresses.
- gether_apply_opts(): Applies the cached options from a struct
  gether_opts to a valid net_device.

To expose these options to userspace, new configfs macros
(USB_ETHER_OPTS_ITEM and USB_ETHER_OPTS_ATTR_*) are defined in
u_ether_configfs.h. These attributes are part of the function
instance's configfs group.

This refactoring is a preparatory step. It allows the subsequent patch
to safely move the net_device allocation from the instance creation
phase to the bind phase without losing the ability to pre-configure
the interface via configfs.

Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
Link: https://patch.msgid.link/20251230-ncm-refactor-v1-1-793e347bc7a7@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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