wil6210: remove 'freq' debugfs

This is completely racy, remove it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220506095451.9852305a92c8.I05155824a1b9059eb59beccde81786dc69de354a@changeid