net: prevent ISA drivers from building on PPC32

Prevent drivers from building on PPC32 if they use isa_bus_to_virt(),
isa_virt_to_bus(), or isa_page_to_bus(), which are not available and
thus cause build errors.

../drivers/net/ethernet/3com/3c515.c: In function 'corkscrew_open':
../drivers/net/ethernet/3com/3c515.c:824:9: error: implicit declaration of function 'isa_virt_to_bus'; did you mean 'virt_to_bus'? [-Werror=implicit-function-declaration]

../drivers/net/ethernet/amd/lance.c: In function 'lance_rx':
../drivers/net/ethernet/amd/lance.c:1203:23: error: implicit declaration of function 'isa_bus_to_virt'; did you mean 'bus_to_virt'? [-Werror=implicit-function-declaration]

../drivers/net/ethernet/amd/ni65.c: In function 'ni65_init_lance':
../drivers/net/ethernet/amd/ni65.c:585:20: error: implicit declaration of function 'isa_virt_to_bus'; did you mean 'virt_to_bus'? [-Werror=implicit-function-declaration]

../drivers/net/ethernet/cirrus/cs89x0.c: In function 'net_open':
../drivers/net/ethernet/cirrus/cs89x0.c:897:20: error: implicit declaration of function 'isa_virt_to_bus'; did you mean 'virt_to_bus'? [-Werror=implicit-function-declaration]

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: David S. Miller <davem@davemloft.net>