staging: r8188eu: remove the HW_VAR_CHECK_TXBUF "hal variable"

Setting HW_VAR_CHECK_TXBUF is an empty operation.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210918141034.28481-1-martin@kaiser.cx


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