usb: gadget: f_hid: added GET_IDLE and SET_IDLE handlers

usb: gadget: f_hid: added GET_IDLE and SET_IDLE handlers

The USB HID standard declares mandatory support for GET_IDLE and SET_IDLE
requests for Boot Keyboard. Most hosts can handle their absence, but others
like some old/strange UEFIs and BIOSes consider this a critical error
and refuse to work with f_hid.

This primitive implementation of saving and returning idle is sufficient
to meet the requirements of the standard and these devices.

Acked-by: Felipe Balbi <balbi@kernel.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Maxim Devaev <mdevaev@gmail.com>
Link: https://lore.kernel.org/r/20210721180351.129450-1-mdevaev@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>