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 <[e-mail]>

arch/avr32/lib/io-readsb.S

···
 	sub	r10, 1
 	st.b	r11++, r8
 	brne	3b
+
+	retal	r12