b43: Fix ofdmtab write regression

commit f04b3787bbce4567e28069a9ec97dcd804626ac7 introduced
a regression for the ofdmtable writing.

It incorrectly removed the writing of the high 16bits for
a 32bit table write and initialized the direction identifier
too late.

This patch does also some cleanups to make the code much more
readable and adds a few comments, so non rocket scientists are
also able to understand what this address caching is all about.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>