rt2x00: Enable WLAN LED on Ralink SoC (rt305x) devices

This patch adds WLAN LED support to the mac80211 rt2x00 driver for
Ralink SoC (rt305x) devices.  The current WLAN LED drivers in
rt2800lib.c set the LED brightness via an MCU request, but do nothing
for SoC.  This patch checks for SoC and sets the register to enable the
WLAN LED (instead of an MCU request).  This enables the WLAN LED for
RT305x devices.

Signed-off-by: Layne Edwards <ledwards@astrumtech.net>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>