brcmsmac: Remove always false 'channel < 0' statement

As 'channel' is declared as u16, the following expression is always false.
   channel < 0

So we can remove unnecessary 'always false' statement.

Signed-off-by: Austin Kim <austindh.kim@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>