ISO15693 device side improvements (#652)

* ISO15693 device side improvements
* increase accuracy by doubling the sample frequency (hi_read_rx_xcorr.v)
* adjust armsrc/iso15693.c and client/cmdhf15.c accordingly
* use more accurate approximation for sqrt(ci^2 + cq^2)
* improve EOF detection (was often mistaken for Logic0, resulting in "error, uneven octet! (extra bits!)")
* hi_read_r_xcorr.v: avoid overflows during accumulation and truncation
* explicitely cast unsigned ADC samples to signed