brcmfmac: Give priority to 5GHz band in selecting target BSS

When a BSS provides both 2.4GHz and 5GHz bands, in many cases it
makes sense to choose 5GHz. Typically a 5GHz channel is less crowded
and has less interference and therefore its performance will be
better than a crowded 2.4 GHz channel. This patch configures
'join_pref' to induce firmware to preferably select 5GHz BSS.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Signed-off-by: Daniel Kim <dekim@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>