LimeSDR: change tx window type to TX_WINDOW_FIXED

It seems that TX_WINDOW_USRP1 is for devices that do not support tx
sync to timestamp. LimeSDR supports it. Changing to TX_WINDOW_FIXED
greatly reduces number of "dumping stale buffer" messages

Modified to match current master by Harald Welte.

Change-Id: I8de5b165ccd72a62b0f16655618e24ca740d9637
1 file changed