[SCSI] esp_scsi: Make cur_residue and tot_residue signed.

Many of the overflow checks test whether the value has
gone negative, and we want to retain such checks.

Reported by Julia Lawall.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>