scsi: 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.

Change-Id: I90eac425a52e22d3f6fc00b74d32497e7a998c0c
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>