LinuxKPI: 802.11 simplify beacon checks in rx path

In linuxkpi_ieee80211_rx() check if the frame is a beacon once upfront
and use the result for enhanced debugging and further checks.
This was done intially for rx_status->device_timestamp debugging.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit c0cadd99d1e36ffe0994f8b30df2939f1d3d0fee)