ath6kl: wmi: prevent a shift wrapping bug in ath6kl_wmi_delete_pstream_cmd()

The "tsid" is a user controlled u8 which comes from debugfs.  Values
more than 15 are invalid because "active_tsids" is a 16 bit variable.
If the value of "tsid" is more than 31 then that leads to a shift
wrapping bug.

Fixes: 8fffd9e5 ("ath6kl: Implement support for QOS-enable and QOS-disable from userspace")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200918142732.GA909725@mwanda