AVR32: Add missing return instruction in __raw_writesb

__raw_writesb ends with a conditional branch, which is obviously
wrong. It should return after the last loop terminates.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>

arch/avr32/lib/io-readsb.S

@@ -45,3 +45,5 @@ __raw_readsb:
 	sub	r10, 1
 	st.b	r11++, r8
 	brne	3b
+
+	retal	r12