wifi: ath12k: dp_mon: Fix unsigned comparison with less than zero

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

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=4060


Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230215011453.73466-1-yang.lee@linux.alibaba.com