staging: r8188eu: simplify the calculation of ie start offset

Use offsetof to calculate the start offset of the information elements in
an association response message. This should make it easier to understand
how the offset is calculated.

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/20220806111352.690650-6-martin@kaiser.cx


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