staging: rtl8192e: use struct pointer to get the size of the struct

Use pointer to the structure to get the size of the structure in order
to conform to the Linux kernel coding style. Issue found using
checkpatch.

Signed-off-by: Santha Meena Ramamoorthy <santhameena13@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>