m68k/mac: Make SCC reset work more reliably

For SCC initialization we cannot assume that the control register is in
the correct state to accept a register pointer. So first read from the
control register in order to "sync" up.
Signed-off-by: NFinn Thain <fthain@telegraphics.com.au>
Signed-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/kernel/head.S

@@ -2915,7 +2915,9 @@ func_start	serial_init,%d0/%d1/%a0/%a1
 
 #if defined(MAC_USE_SCC_A) || defined(MAC_USE_SCC_B)
 	movel	%pc@(L(mac_sccbase)),%a0
-	/* Reset SCC device */
+	/* Reset SCC register pointer */
+	moveb	%a0@(mac_scc_cha_a_ctrl_offset),%d0
+	/* Reset SCC device: write register pointer then register value */
 	moveb	#9,%a0@(mac_scc_cha_a_ctrl_offset)
 	moveb	#0xc0,%a0@(mac_scc_cha_a_ctrl_offset)
 	/* Wait for 5 PCLK cycles, which is about 68 CPU cycles */