rt2x00: Remove incorrect led blink

rt2800 devices are not capable of configuring arbitrary LED on/off
periods. The LED_CFG register fields ON_PERIOD and OFF_PERIOD are only
used by the hw when the LED mode is set to "blink upon TX".

Hence, remove the blink callback. This will result in software emulation
for LED blinking.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>