ssb: fix per path sprom vars

On sprom version 4 and 5 there are 4 values for pa_2g, pa_5gl, pa_5g
and pa_5gh, for sprom version 8 and 9 there are only 3. Make the per
path sprom store also work for older sprom versions.

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