rt2800: correct BBP1_TX_POWER_CTRL mask

Two bits control TX power on BBP_R1 register. Correct the mask,
otherwise we clear additional bit on BBP_R1 register, what can have
unknown, possible negative effect.

Cc: stable@vger.kernel.org
Signed-off-by: NStanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2800.h

@@ -2041,7 +2041,7 @@ struct mac_iveiv_entry {
  * 2 - drop tx power by 12dBm,
  * 3 - increase tx power by 6dBm
  */
-#define BBP1_TX_POWER_CTRL		FIELD8(0x07)
+#define BBP1_TX_POWER_CTRL		FIELD8(0x03)
 #define BBP1_TX_ANTENNA			FIELD8(0x18)
 
 /*