si4700: optimize RDS cache update a little

use si4700_read() instead of si4700_read_reg() to make it clear we are
not interested in the register itself but in the cache update. Also, a
tiny bit more efficient as we save a function call and don't request a
return value we don't use anyhow.

Change-Id: Ibbb66fd9e5ea748ba11ac3c2a0570f5219b72637

firmware/drivers/tuner/si4700.c

@@ -586,7 +586,7 @@ void si4700_rds_process(void)
 
     if (tuner_powered())
     {
-        si4700_read_reg(RDSD);
+        si4700_read(6);
 #if (CONFIG_RDS & RDS_CFG_POLL)
 	/* we need to keep track of the ready bit because it stays set for 80ms
 	 * and we must avoid processing it twice */