staging: vt6655: vnt_bss_info_changed check conf->beacon_rate is not NULL

conf->beacon_rate can be NULL on association. So check conf->beacon_rate

BSS_CHANGED_BEACON_INFO needs to flagged in changed as the beacon_rate
will appear later.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Cc: <stable@vger.kernel.org> # v3.19+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>