swim: Add missing spinlock init

It doesn't seem this spinlock was properly initialized.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Finn Thain <fthain@telegraphics.com.au>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>