For an unkown reason (at moment), sometimes if_cpsw cannot read from PHY
and fails to attach calling cpsw_detach() which end up in a panic.
Fix it by doing the proper check before detach the miibus and also fix the
leak of few variables.
And to actually make it work, ether_ifattach() has to be moved to the end
of cpsw_attach() to avoid a race where calling ether_ifdetach() before
domain_init() (which will only run later on) would make it crash at
INP_INFO_RLOCK() on in_pcbpurgeif0().
Tested on: BBB (am335x)
MFC after: 1 week
c2bbf09479