staging: r8188eu: read aid from struct ieee80211_mgmt

Read the aid of the association response message from struct
ieee80211_mgmt. This should be easier to understand and to review than
calculating the offset manually.

Remove the cast to int, aid is a u16.
Keep the 0x3fff mask that is currently applied to the aid.

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220806111352.690650-5-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>