usb: hid: default request handlers

HID class now defaults to universal request callbacks in case they
are not provided by the application. This applies to following,
class-specific requests: Get_Report, Set_Report, Get_Idle, Set_Idle,
Get_Protocol and Set_Protocol.
Tested with USB3CV.

Signed-off-by: Marcin Szymczyk <Marcin.Szymczyk@nordicsemi.no>