Channel mask works incorrect with xSDR@usb

[irodushka]

Conditions:

1. xSDR on dev board (green V1)
2. USB wiring
3. Transmitting IQ file with usdr_dm_create (carrier 390M, modulation 1kHz, sr 1M)
   ./usdr_dm_create -t -r1e6 -c-1 -Y4 -E390e6 -e390e6 -I ./signal_1khz.ci16 -C<varies> -o
4. Using different chmask (0b01, 0b10, 0b11) to test RF channels separation

Expecting:

1. Transmission via ch#1 only when chmask == 0b01
2. Transmission via ch#2 only when chmask == 0b10
3. Transmission through ch#1 & ch#2 when chmask == 0b11

Result:

1. When chmask == 0b01 or 0b10 - "good" transmission via ch#1 & ch#2 (carrier + modulation)
2. When chmask == 0b11 - "poor" transmission via ch#1 & ch#2 (carrier only, no modulation)
3. Cannot achieve the situation when the channels work separately.