staging: rtl8723au: Use kernel memory allocator for sta_priv allocations

The kernel has a great memory allocator, use it instead of hacking up
something in the driver.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>