brcmfmac: Fix reconnect failure after beacon timeout

The DISASSOC command needs to be sent to firmware when a connection
loss is detected by firmware (e.g., beacon timeout). Otherwise the
next connect request fails due to a lingering LINK(down) event from
firmware. This patch resolves the issue by using brcmf_link_down()
handler, instead of the incomplete duplicated codes.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@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>