ar9170: small misc changes

This patch aggregates a bunch of small random changes
that won't fit really anywhere else properly.

1. move tid-locating macro into a separate function.

2. remove redundant NULL check.

3. add modulation mask definition

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>