net/irda: sh_sir: fixup err return value on sh_sir_open

On sh_sir_open function, there was a possibility that
err variable didn't have value even though it is return value.
This patch modify it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>