ssb: extif: fix compile errors

If CONFIG_SSB_EMBEDDED or CONFIG_SSB_DRIVER_MIPS is set and
CONFIG_SSB_DRIVER_EXTIF is not set, it will cause compile problems
because of missing functions. This patch fixes these problems.

The mips driver now also uses ssb_chipco_available() instead of
checking bus->chipco.dev manually.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>