net: sun: Remove unneeded cast from memory allocation

Remove casting the values returned by memory allocation function.

Coccinelle emits WARNING: casting value returned by memory allocation
function to (struct cas_init_block *) is useless.

This issue was detected by using the Coccinelle software.

Signed-off-by: Aishwarya Ramakrishnan <aishwaryarj100@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>