ppc4xx: Fix problem in 4xx_enet.c driver

U-Boot crashes in the net loop if CONFIG_4xx_DCACHE is
enabled. To reproduce the problem ensure that 'ethrotate'
environment variable isn't set to "no" and then run
"tftp 200000 not_existent_file".
This patch tries to fix the issue.

Signed-off-by: Anatolij Gustschin <agust@denx.de>