brcmfmac: Avoid possible out-of-bounds read

In brcmf_p2p_notify_rx_mgmt_p2p_probereq(), chanspec is assigned before
the length of rxframe is validated.  This could lead to uninitialized
data being accessed (but not printed).  Since we already have a
perfectly good endian-swapped copy of rxframe->chanspec in ch.chspec,
and ch.chspec is not modified by decchspec(), avoid the extra
assignment and use ch.chspec in the debug print.

Suggested-by: Mattias Nissler <mnissler@chromium.org>
Signed-off-by: Kevin Cernekee <cernekee@chromium.org>
Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>