staging: sbe-2t3e3: remove t3e3_tx_desc_t typedef

Kernel coding style does not recommend using typedefs for structures, so
remove the t3e3_tx_desc_t typedef.

Fix the following checkpatch warning:
drivers/staging/sbe-2t3e3/2t3e3.h:651: WARNING: do not add new typedefs

Signed-off-by: Kristina Martšenko <kristina.martsenko@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>