Staging: rtl8712: rtl8712_xmit: Remove useless cast

Explicit typecasting of variables is not required and should be removed.
Semantic patch used:
@@
type T;
T e;
identifier x;
@@

* T x = (T)e;

Signed-off-by: Shivani Bhardwaj <shivanib134@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>