bcm2835-sdhost: Improvements to error recovery

1) Try to avoid reducing overclock when a card is removed.

2) Reset overclock on card insertion.

3) Reduce logging when errors occur, lowering the severity of
   some messages and making others conditional on the debug
   flag.

4) Attempt to identify a disconnected SD bus earlier, treating a
   zero returned OCR (voltage support) as an error condition.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>