staging: r8188eu: tx_retevt semaphore is not used

Remove tx_retevt from struct xmit_priv. This semaphore is initialised but
not used.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Link: https://lore.kernel.org/r/20221230180646.91008-17-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>