Staging: rtl8192e: Use struct_size

Using struct_size is encouraged because it is safer
than using sizeof and calculations by hand.

Signed-off-by: Felix Schlepper <f3sch.git@outlook.com>
Link: https://lore.kernel.org/r/54ea62a2b7628d3bdd5aa52df82c3e91704a2127.1656078068.git.f3sch.git@outlook.com


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