staging: rtl8192e: rtl8192e: Remove NULL test before vfree

vfree frees the virtually continuous block of memory beginning at addr.
If addr is NULL, no operation is performed. So, NULL test is not needed
before vfree.

Signed-off-by: Shyam Saini <mayhs11saini@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>