staging: r8712u: Fix macros used to read/write the TX/RX descriptors

Although the driver works on big-endian hardware, Sparse generates a lot
of warnings. Many of these are the result of incorrect coding of these
macros.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>