staging: r8188eu: read capability info from struct ieee80211_mgmt

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

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/20220806111352.690650-4-martin@kaiser.cx


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