usb: gadget: hid: allow dynamic interval configuration via configfs

This patch enhances the HID gadget driver to support dynamic configuration
of the interrupt polling interval (bInterval) via configfs.  A new
‘interval’ attribute is exposed under each HID function’s configfs
directory, and any write to it will adjust the poll rate for all endpoints
without requiring a rebuild.

When the attribute has never been written, legacy defaults are preserved:
  • Full-Speed (FS) endpoints (IN & OUT) poll every 10 ms
  • High-Speed (HS) endpoints (IN & OUT) poll every 4 micro-frames
    (~1 ms)

To implement this cleanly:
  • Add two new fields to f_hid_opts and f_hidg:
      – unsigned char interval
      – bool           interval_user_set
  • Introduce dedicated f_hid_opts_interval_show/store functions.
    The store routine parses into an unsigned int, bounds‐checks,
    assigns to opts->interval, and sets opts->interval_user_set = true.
  • Initialize opts->interval = 4 and opts->interval_user_set = false in
    hidg_alloc_inst(), then copy both into the live f_hidg instance in
    hidg_alloc().
  • In hidg_bind(), set each endpoint’s bInterval based on whether the
  user has written the attribute:
      – If interval_user_set == false, use FS=10 / HS=4
      – If interval_user_set == true, use the user’s value for both FS
        & HS

Signed-off-by: Ben Hoff <hoff.benjamin.k@gmail.com>
Link: https://lore.kernel.org/r/20250429182809.811786-1-hoff.benjamin.k@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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