staging: rtl8188eu: fix signal strength indication

RTL8188E can only have a maximum of 2 chains so match that in the actual
phy_rx_agc_info structure within phy_status_rpt.  This will cause the
other data received from the PHY, such as signal strength indication of
beacons, to properly align and allow extraction and use within the
signal strength record-keeping mechanisms.

Signed-off-by: Andrew Bradford <andrew@bradfordembedded.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>