iwlwifi: pnvm: don't kmemdup() more than we have

We shouldn't kmemdup() more data than we have, that might
cause the code to crash. Fix that by updating the length
before the kmemdup.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20211016114029.ab0e64c3fba9.Ic6a3295fc384750b51b4270bf0b7d94984a139f2@changeid