rt2x00: Restrict interface between rt2x00link and drivers

Restrict drivers to only access link_qual structure during
link tuning. The contents of these fields are for the drivers
and all fields are allowed to be changed to values the driver
considers correct.

This means that some fields need to be moved outside of this
structure to restrict access only to rt2x00link itself.
This allows some code to be moved outside of the rt2x00.h header
and into rt2x00link.c.

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