staging: r8188eu: last_rx_mgnt_pkts is set but not used

last_rx_mgnt_pkts in struct stainfo_stats is set but not used.
Remove it.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220515155401.352742-5-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>