ssb/bcma: fill attribute alpha2 from sprom

ssb/bcma: fill attribute alpha2 from sprom

The attribute country_code and alpha2 are two different attributes in
the sprom. country_code contains some code in an 8 bit coding and
alpha2 contains two chars with the country code. The attributes where
read out wrongly in the past and country_code is only available on
sprom version 1.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Tested-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>