cmd/ethsw: Disable implicit enum conversion warning

cmd/ethsw: Disable implicit enum conversion warning

With clang-3.8 we see warnings like:
cmd/ethsw.c:304:6: warning: implicit conversion from
      enumeration type 'enum ethsw_keyword_opt_id' to different enumeration type
      'enum ethsw_keyword_id' [-Wenum-conversion]
                                        ethsw_id_pvid_no,
                                        ^~~~~~~~~~~~~~~~

Because we have one enum for ethsw_keyword_id and a second enum for
ethsw_keyword_opt_id.  This ends up being safe as ethsw_keyword_opt_id
explicitly starts after ethsw_keyword_id enum ends.   Disable the
warning here rather than collapse these into one enum and rely on
comments to denote where optional keywords begin.

Cc: Codrin Ciubotariu <codrin.ciubotariu@freescale.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>