scsi: aacraid: rcode is unsigned and should be signed int

aac_fib_send can return -ve error returns and hence rcode should be
signed. Currently the rcode >= 0 check is always true and -ve errors are
not being checked.

Thanks to Dan Carpenter for spotting my original broken fix to this
issue.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Raghava Aditya Renukunta <RaghavaAditya.Renukunta@microsemi.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>