brcmfmac: add hexadecimal trace of message payload

Adds a trace function used in brcmf_dbg_hex_dump() which adds the
raw binary data to the trace. It requires trace-cmd plugin to see
this data.

Reviewed-by: Piotr Haber <phaber@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>