icache: Reset FSM to FLUSH

Created by: niwis

@suehtamacv encountered this unexpected behavior. The current implementation resets the icache FSM to IDLE, where the icache checks if it was just enabled (rising edge of en_i), and flushes (initialises) itself if so. This assumes that en_i is deasserted during reset, which is not necessarily the case (e.g. hardwired en_i), possibly leaving the cache uninitialised. Hence, reset to FLUSH directly.