drivers/net/4xx_enet.c: Fix build warning

Fix:
4xx_enet.c: In function 'ppc_4xx_eth_initialize':
4xx_enet.c:2029: warning: assignment from incompatible pointer type

Tested on sequoia board.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Tested-by: Anatolij Gustschin <agust@denx.de>
Cc: Joe Hershberger <joe.hershberger@gmail.com>