ssb: get mac address from sprom struct for gige driver

The mac address is already stored in the sprom structure by the
platform code of the SoC this Ethernet core is found on, it just has to
be fetched from this structure instead of accessing the nvram here.
This patch also adds a return value to indicate if a mac address could
be fetched from the sprom structure.
When CONFIG_SSB_DRIVER_GIGE is not set the header file now also declares
ssb_gige_get_macaddr().

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Michael Buesch <m@bues.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>