Spectral scan rssi offset argument above 15 is not correct

[reissjason]

It appears the the positive value will lower the reading. -16 and 16 have equivalent results.

The argument is read into a 8-bit signed integer and passed to the loragw code.
The value is converted to 2's complement before being set into the FPGA register.
What happens with it after that is in the FPGA code.

/libloragw/src/loragw_radio.c:335
RssiOffsetReg = (rssi_offset >= 0) ? (uint8_t)rssi_offset : (uint8_t)(~(-rssi_offset)+1); /* 2's complement */

admin@mtcdt:/opt/lora/gateway-utils# ./util_spectral_scan -f 902:0.05:903 -n 10000 -o 15
+++ Start spectral scan of LoRa gateway channels +++
...
902000000 10%<-116.5 30%<-117.0 50%<-117.5 80%<-118.0
902050000 10%<-116.5 30%<-117.0 50%<-117.5 80%<-118.0
+++ Exiting Spectral scan program +++
admin@mtcdt:/opt/lora/gateway-utils# ./util_spectral_scan -f 902:0.05:903 -n 10000 -o -15
+++ Start spectral scan of LoRa gateway channels +++
...
902000000 10%<-86.5 30%<-87.0 50%<-87.5 80%<-88.0
902050000 10%<-86.5 30%<-87.0 50%<-87.5 80%<-88.0
admin@mtcdt:/opt/lora/gateway-utils# ./util_spectral_scan -f 902:0.05:903 -n 10000 -o 16
+++ Start spectral scan of LoRa gateway channels +++
...
902000000 10%<-85.5 30%<-86.0 50%<-86.5 80%<-87.0
902050000 10%<-85.5 30%<-86.0 50%<-86.5 80%<-87.0
admin@mtcdt:/opt/lora/gateway-utils# ./util_spectral_scan -f 902:0.05:903 -n 10000 -o -16
...
902000000 10%<-85.5 30%<-86.0 50%<-86.5 80%<-87.5
902050000 10%<-85.5 30%<-86.0 50%<-86.5 80%<-87.0

[rbaldwin13]

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