s2io.c: Use calculated size in kmallocs

Use consistent style.  Don't calculate the kmalloc size multiple times

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Sreenivasa Honnur <sreenivasa.honnur@neterion.com>
Signed-off-by: David S. Miller <davem@davemloft.net>