rt2x00: Fix RF offset

The word_base is in bytes instead of word index number,
this means that when using it, it should be transformed into
a word index first.

Otherwise RF register reading will fail through debugfs since
we would start reading 4 words starting with word 4 (which is the last
valid word for RF).

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>