juntek-jds6600: speedup serial communication, reduce timeouts

juntek-jds6600: speedup serial communication, reduce timeouts

Wire traffic captures for serial-over-USB communication between the host
and the signal generator suggests that: Device firmware's reponses are
immediate. The 115200 UART is the limiting factor, USB FS adds little.

Comment on these findings for later reference. Reduce timeouts and
remove delays where appropriate. This dramatically speeds up the scan
for the device, and subsequent communication. Identification and getting
the current configuration takes a total of 70ms after this change.