qtnfmac: fix invalid channel information output

commit 24227a9e

 upstream.

Do not attempt to print frequency for an invalid channel
provided by firmware. That channel may simply not exist.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>