staging: rtl8192e: Remove unnecessary code

kfree on NULL pointer is a no-op.

This used the following semantic patch to identify such a instance

// <smpl>
@@ expression E; @@
- if (E != NULL) { kfree(E); }
+ kfree(E);
@@ expression E; @@
- if (E != NULL) { kfree(E); E = NULL; }
+ kfree(E);
+ E = NULL;
// </smpl>

As rxb is always null at this point, so the code to kfree it and intializing
it to NULL is removed completely.

Suggested by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Tapasweni Pathak <tapaswenipathak@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>