ath11k: Drop multicast fragments

Fragmentation is used only with unicast frames. Drop multicast fragments
to avoid any undesired behavior.

Tested-on: IPQ8074 hw2.0 AHB WLAN.HK.2.4.0.1-01734-QCAHKSWPL_SILICONZ-1 v2

Cc: stable@vger.kernel.org
Signed-off-by: Sriram R <srirrama@codeaurora.org>
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Link: https://lore.kernel.org/r/20210511200110.1d53bfd20a8b.Ibb63283051bb5e2c45951932c6e1f351d5a73dc3@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>