rt2x00: Fix vgc_level_reg handling

Currently vgc_level_reg and vgc_level are equal to eachother,
while the purpose of vgc_level_reg is the value last written
to the register and is remembered through link tuning resets.
The vgc_level is the currently active level, which is
reset during link tuning resets.

The usage of these variables depends on the drivers, some drivers
need both, while others need only one of the two.

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