carl9170: Clarify kconfig text

The previous text was confusing, leading readers to think this
driver was a duplicate, and so didn't need to be enabled.

After the removal of the older staging driver, this is the only
driver in mainline for these devices.

Signed-off-by: Lauri Kasanen <cand@gmx.com>
Acked-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>