staging: rtl8188eu: core: 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>