wifi: iwlwifi: mvm: don't access packet before checking len

Currently in sniffer mode we access pkt fields before checking that
the frame has the length to access it. Fix this by moving the check
to before the access.

Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20221205102808.934da230c698.Ib56f11bbc8978e15d38394336a929cb4996ba39e@changeid