Read-only support.

This patch adds a new callback for the drivers to facilitate reading the 
SD card read-only switch. If the callback is not provided then a warning 
will be printed and it will default to write-enable.

The read-only switch is a host enforced read-only so the MMC block layer 
has been changed to not allow rw mounts of ro cards. It also prints a 
'(ro)' for read-only cards.