[SCSI] sd: Always do write-protect check

Since nobody has offered an explanation for why the sd driver makes a
write-protect check only for devices with removable media, I'm submitting
this patch to get rid of the removable-media test.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>