bcm2836sdhc unload

[idigdoug]

Both sdport.sys and bcm2836sdhc.sys have bugs that prevent them from successfully unloading. Fix the bug in bcm2836sdhc.sys so that it will be able to unload correctly once sdport.sys gets fixed.

bcm2836sdhc:

• Add a Cleanup handler. This is not optional -- sdport.sys does not check for NULL before trying to call it, and it will be called every time sdport.sys tries to unload the driver. In addition, if it doesn't clean up WPP, we'll crash shortly after unloading.
• Add a PowerControlCallback handler. This is not optional -- sdport.sys does not check for NULL before trying to call it. This is less of an issue because I've never actually seen sdport try to call this.
• Clean up WPP if SdPortInitialize fails.

rpisdhc:

• Add a PowerControlCallback handler. This is not optional -- sdport.sys does not check for NULL before trying to call it. This is less of an issue because I've never actually seen sdport try to call this.
• Clean up WPP if SdPortInitialize fails.

[mariobalanica]

Thanks!