iwlwifi: avoid crash on unsupported debug collection

If the opmode doesn't support debug collection (DVM) then don't
crash, but just skip the callback.

Fixes: d0129315 ("iwlwifi: dbg: add op_mode callback for collecting debug data.")
Reported-by: Andy Lavr <andy.lavr@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210219133506.ecabe285bc7d.I73d230d555c595fa2d9bf284f80078729fe18aa4@changeid