b43: remove warning for LP-PHY with sprom < 8

The BCM5354 SoC has a build in ieee80211 core rev 13 with a LP-PHY on
it. This devices has a sprom version 3 stored in the nvram. This patch
removes the warning and uses the opo values from the sprom as mentioned
in the specs.

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