brcmutil: print invalid chanspec when WARN-ing

On one of my devices I got WARNINGs when brcmfmac tried to decode
chanspec. I couldn't tell if it was some unsupported format or just a
malformed value passed by a firmware.

Print chanspec value so it's possible to debug a possible problem.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>