Fix another inconsistency in the naming of fields

"rx_freq" should be renamed to "frequency" to remain consistent with the
Transmitter test command.
diff --git a/lib/hci.h b/lib/hci.h
index 7e3e328..86071bb 100644
--- a/lib/hci.h
+++ b/lib/hci.h
@@ -1485,7 +1485,7 @@
 
 #define OCF_LE_RECEIVER_TEST			0x001D
 typedef struct {
-	uint8_t		rx_freq;
+	uint8_t		frequency;
 } __attribute__ ((packed)) le_receiver_test_cp;
 #define LE_RECEIVER_TEST_CP_SIZE 1