sd: Avoid sending medium write commands if device is write protected

The SYNCHRONIZE_CACHE command is a medium write command and hence can
fail when the device is write protected. Avoid sending such commands by
making sure that write-cache-enable is disabled even though the device
claim to support it.

Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>
Signed-off-by: Dolev Raviv <draviv@codeaurora.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Venkatesh Srinivas <venkateshs@google.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>