card_emu: waiting_time is stored in etu, not clocks.

The comment didn't reflect the source.  I checked all users and
the code consistently stores the waiting time in units of 'etu'.

Change-Id: I2bc4a7c19cee5fb487ad639ee48ecaea706f6172

firmware/libcommon/source/card_emu.c

@@ -165,7 +165,7 @@ struct card_handle {
 	uint8_t in_ep;		/* USB IN EP */
 	uint8_t irq_ep;		/* USB IN EP */
 
-	uint32_t waiting_time;	/* in clocks */
+	uint32_t waiting_time;	/* in etu */
 
 	/* ATR state machine */
 	struct {