forcedeth: Separate vendor specific initializations into functions

Neaten the phy_init function by adding and calling vendor
specific functions.

object size is reduced by ~1kb:

$ size drivers/net/forcedeth.o.*
   text	   data	    bss	    dec	    hex	filename
  83475	   1848	  19304	 104627	  198b3	drivers/net/forcedeth.o.new
  84459	   1848	  19544	 105851	  19d7b	drivers/net/forcedeth.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>