staging: r8188eu: read reason code from ieee80211_mgmt

Use the struct ieee80211_mgmt to read the reason code instead of parsing
the message manually.

Remove the pframe pointer, it's no longer used.

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/20221129205152.128172-3-martin@kaiser.cx


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