[SCSI] Legacy MegaRAID: Fix READ CAPACITY

Some Legacy megaraid cards can't actually cope with the scatter/gather
version of the READ CAPACITY command (which is what we now send them
since altering all SCSI internal I/O to go via the block layer).  Fix
this (and a few other broken megaraid driver assumptions) by sending
the non-sg version of the command if the sg list only has a single
element.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>