LinuxKPI: 802.11: enable fragmentation offload

Let the hardware handle fragmentation itself and tell net80211 to
save itself from it. We already call the (*set_frag_threshold)
mac80211 function.

This should be a NOP for all currently enabled LinuxKPI based WiFi
drivers.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 63578bf225df37944b78febfb177e8c1c81f54e4)