iwlwifi: mvm: do not print security error in monitor mode

In monitor mode we are not expected to decrypt encrypted
packets (not having the keys).
Hence we are expected to get an unknown rx security status.
Keeping the print in monitor mode causes a print for each
captured packet flooding the dmesg.

Signed-off-by: Shaul Triebitz <shaul.triebitz@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>