powerpc: use asm-generic/socket.h as much as possible

asm-generic/socket.h already has an exception for the differences that
powerpc needs, so just include it after defining the differences.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>