staging:rtl8192u: Remove struct rx_desc_819x_usb_aggr_subframe

Removal of structure rx_desc_819x_usb_aggr_subframe from local header file,
which is not used outside the header file.

Signed-off-by: John Whitmore <johnfwhitmore@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>