Fix for "issues/62 fix PLL1 overclock bug" see hackrf_core.c -> cpu_cloc...

...k_init()

Fix for "issues/78 startup current too high" see hackrf_core.c -> New functions cpu_clock_pll1_low_speed()/cpu_clock_pll1_max_speed() & hackrf_usb.c to switch low_speed/max_speed.