staging: r8188eu: calculate the delba length

Use offsetofend to calculate the length of the delba message.

We are now calculating all message lengths based on the offset of their
last field. All intermediate updates of pattrib->pktlen can be removed.

Tested-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220602193726.280922-10-martin@kaiser.cx


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