rt2x00: Fix Sparse warning

rt2x00link_reset_qual() is not declared in a header,
and is only internally used within rt2x00link.c.
It should be declared as static.

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