ppc4xx: Fixed compilation warning in 4xx_enet.c

This patch fixes a compilation warning for some Ethernet PHY-less
PPC4xx platforms (440SPE based ones) and a potential compilation
error for 440SP platforms (use of undefined 'ethgroup' variable).
In the original code and in case of 440SPE platforms, 'ethgroup'
is initialized to -1 and never modified.  Later in the function,
within an #ifdef statement, an 'if statement' executes code only
if 'ethgroup' is set to 4, therefore it is harmless to avoid
executing the 'if statement' by removing the CONFIG_440SPE from
the affected #ifdefs.  In case of 440SP platforms  with on-board
Ethernet PHY, 'ethgroup' is undefined but used (there are not such
platforms in the repository yet). All other architectures are not
affected by this change.

Signed-off-by: Alessio Centazzo <acpatin@yahoo.com>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>