wil6210: move vring_idle_trsh definition to wil6210_priv

vring_idle_trsh is used in the operational driver, hence
should not be defined as a debugfs variable.

Signed-off-by: Gidon Studinski <qca_gidons@qca.qualcomm.com>
Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>