b43: HT-PHY: rename defines addressing cores on the 2059 radio

After comparing writes to registers at 0x000, 0x400 and 0x800 it seems
there are many very similar writes. So 0x000 offset is not for accessing
something totally different, but probably just the first out of three
cores.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>