staging: rtl8723au: Sanitize USB read/write functions

The original Realtek provided functions suffered badly from clutter to
accommodate broken operating systems. Lets try this lean and clean
version instead.

v2: Do not use the stack for data passed to usb_control_msg(). This
    requires reintroducing the mutex used in the old function. In
    addition, get rid of the no longer used 'usb_vendor_req_buf'.
    Note that rtl8723au_writeN() remains unlocked, so it can be used
    for bulk block transfers without having to retake the mutex for
    every write().

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