wil6210: Fix division by zero in wil_vring_debugfs_show

On some platforms get_cycles() implemented to allways return 0.
On such platforms "Division by zero" bug was triggered.

Signed-off-by: Boris Sorochkin <boriss@codeaurora.org>
Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>