staging: r8188eu: Remove initialized but unused semaphore

Remove semaphore "xmit_sema" from "struct adapter" because it is
initialized but never used.

Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Link: https://lore.kernel.org/r/20211022101028.30367-2-fmdefrancesco@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>