ath6kl: Array index out of bounds check

The variable assigned_ep can be assigned value of -1 and is never
checked if it equals -1. So the endpoint array can have -1  as the index
value and can be out of bounds.

The value of assigned_ep is checked for -1 and is ensured that the
endpoint array doesn't go out of bounds.

Signed-off-by: Pandiyarajan Pitchaimuthu <c_ppitch@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>