net: wireless: rt2x00: rt2x00mac.c: Cleaning up uninitialized variables

There is a risk that the variable will be used without being initialized.

This was largely found by using a static code analysis program called cppcheck.
Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2x00mac.c

@@ -799,6 +799,8 @@ int rt2x00mac_set_antenna(struct ieee80211_hw *hw, u32 tx_ant, u32 rx_ant)
 
 	setup.tx = tx_ant;
 	setup.rx = rx_ant;
+	setup.rx_chain_num = 0;
+	setup.tx_chain_num = 0;
 
 	rt2x00lib_config_antenna(rt2x00dev, setup);