ppc4xx/POST: Change ethernet test loop count to a default of 10

This patch changes the PPC4xx ethernet POST loop test count from
currently 192 (256 - 64) to a default of 10. While doing this the max
frame size is increased. Each loop run uses a different frame size,
starting with a max of 1514 bytes, down to 64. The default loop
count of 10 can be overriden using CONFIG_SYS_POST_ETH_LOOPS in the
board config header.

The TEST_NUM loop has been removed as it was never used.

The main reason for this change is to reduce the boot time on boards
using this POST test, like the lwmon5 board. This change reduces the
boot time by about 600ms on the lwmon5 board.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Wolfgang Denk <wd@denx.de>