cxgb4/ptp: pass the sign of offset delta in FW CMD

cxgb4_ptp_fineadjtime() doesn't pass the signedness of offset delta
in FW_PTP_CMD. Fix it by passing correct sign.

Signed-off-by: Raju Rangoju <rajur@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>