staging: rtl8712: Remove unnecessary cast for argument of kfree

This patch removes all occurrences of unnecessary cast on the
argument to kfree, all over in rtl8712.

There is no need of the cast on the argument to kfree.

Greped to find occurrences.

Signed-off-by: Tapasweni Pathak <tapaswenipathak@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>