sctp: fix SSN comparision

This function actually operates on u32 yet its paramteres were declared
as u16, causing integer truncation upon calling.

Note in patch context that ADDIP_SERIAL_SIGN_BIT is already 32 bits.

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>