sfc: ethtool: Refactor to remove fallthrough comments in case blocks

Converting fallthrough comments to fallthrough; creates warnings
in this code when compiled with gcc.

This code is overly complicated and reads rather better with a
little refactoring and no fallthrough uses at all.

Remove the fallthrough comments and simplify the written source
code while reducing the object code size.

Consolidate duplicated switch/case blocks for IPV4 and IPV6.

defconfig x86-64 with sfc:

$ size drivers/net/ethernet/sfc/ethtool.o*
   text	   data	    bss	    dec	    hex	filename
  10055	     12	      0	  10067	   2753	drivers/net/ethernet/sfc/ethtool.o.new
  10135	     12	      0	  10147	   27a3	drivers/net/ethernet/sfc/ethtool.o.old

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