brcmfmac: make chandef_to_chanspec() function static

The function chandef_to_chanspec() was added by

    brcmfmac: determine chanspec from struct cfg80211_chan_def info

    The struct cfg80211_chan_def contains additional info to derive the
    bandwidth and side-band information of the chanspec. This patch adds
    chandef_to_chanspec() function used in IBSS join and starting AP
    operation.

However, it introduced a sparse warning because the function
is only called from within the source file wl_cfg80211.c.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Reviewed-by: Daniel (Deognyoun) Kim <dekim@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>