improve SD cardreader power management

this reverts our current solution that changes the global scsi error handling, which is tricky and hard to maintain.

instead, use an approach that is at least more likely to be accepted upstream: properly add a kernel-internal flag for cardreaders and only handle the problem in the sd driver. The solution was suggested by Bart Van Assche a few months ago and seems to work.

I need you to test this though ( cat /sys/bus/usb/devices/1-1/power/runtime_status while mounting / using / unmounting an SD card)

This shouldn't change anything! It's still a great improvement in maintainability and helps with mainlining. thanks for testing!