iwlwifi: mvm: beacon statistics shouldn't go backwards

We reset statistics also in case that we didn't reassoc so in
this cases keep last beacon counter.

Cc: stable@vger.kernel.org # v4.19+
Signed-off-by: Mordechay Goodstein <mordechay.goodstein@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.20200417100405.1f9142751fbc.Ifbfd0f928a0a761110b8f4f2ca5483a61fb21131@changeid