ravb: fix result value overwrite

The result value is overwritten by a return value of
ravb_ptp_interrupt().

Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>