The return value from the call to idr_alloc() is int.
However, the return value is being assigned to an unsigned
int variable 'buf_id', so making 'buf_id' an int.
Eliminate the following warning:
./drivers/net/wireless/ath/ath12k/dp_mon.c:1300:15-21: WARNING: Unsigned expression compared with zero: buf_id < 0