staging: r8188eu: silentreset_mutex is unused

silentreset_mutex from struct sreset_priv is not used. Remove it.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20211024180448.20624-4-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>