staging: brcm80211: replaced 5Ghz specific wf_channel2mhz()

Code cleanup related. Replaced broadcom specific function with Linux function
ieee80211_dsss_chan_to_freq().

Reviewed-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>