bug.h: introduce WARN_ONCE
    
    Add WARN_ONCE definition to allow single time notification
    of warnings to the user.
    Taken from Linux kernel (4.17) with slight changes
    (Removed __section(.data.once))
    
    Signed-off-by: Ramon Fried <ramon.fried@gmail.com>
    [trini: Drop the musb and dwc3 compat versions]
    Signed-off-by: Tom Rini <trini@konsulko.com>