[SCSI] aacraid: Fix return code interpretation

Received from Mark Salyzyn

clear_user return is 0 for success, the code fragment is written to
assume that it is the count of the number of bytes zero'd.

Signed-off-by: Mark Haverkamp <markh@osdl.org>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>