MII/RMII buffering uses a global for the lock meaning lib is not re-entrant

[Here](https://github.com/xmos/lib_ethernet/blob/e6ab77e2a0fbb920602bb4e2acbc74bc9028b3f2/lib_ethernet/src/mii_buffering.c#L27) is the offending line.

Should anyone want two instances on the same tile (which is possible - see RMII bringup [app](https://github.com/xmos/lib_ethernet/blob/develop/tests/bringup_xk_eth_xu316_dual_100m/src/main.xc)) it may cause problems when pushing the MAC.