[SCSI] gdth: Stop abusing ->done for internal commands

The ->done member was being used to mark commands as being internal.
I decided to put a magic number in ->underflow instead.  I believe this
to be safe as no current user of ->underflow has any of the bottom 9
bits set.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>