Solve the SI570 frequency update issues

Code modifications proposed to bdaq53.v and bdaq53_core.v to guarantee the execution of updating the si570 frequency from 156.25 MHz to 160 MHz only once in BDAQ board, even if the the software sends other extra frequency update commands. This avoids the problem of losing the RD53 correct configuration witnessed from time to time, using Bdaq and also Yarr software. Resetting (or power cycling) the BDAQ53 hardware enables again the frequency updating once.