[net80211] refactor out "add slot" and "purge slot" for A-MPDU.

This is in preparation for A-MSDU decap in A-MPDU support.

• refactor out the code to purge a single reorder slot into ampdu_rx_purge_slot().
• refactor out the code to add a frame to the given reorder slot to ampdu_rx_add_slot().

This should be a big no-op as far as current code is concerned.

Tested:

• QCA9880v2, STA mode (11ac)
• iwn(4), STA mode (11n)

Reviewed by: avos
Differential Revision: https://reviews.freebsd.org/D10328