[Blackfin] arch: fix bug - a crash on bootup with CONFIG_MPU on the BF548

The function flush_switched_dcplbs was clearing the CPLB entries covering
the process permission bitmasks.  This means that the sequence

	flush_switched_dcplbs ();
        set_mask_dcplbs(mm->context.page_rwx_mask);

has a problem: if kernel code (such as an interrupt) causes a CPLB miss before
set_mask_dcplbs completes, the CPLB handler function causes a double fault,
with an instantaneous reboot.

This bug fix is dedicated to Michael Hennerich, the only person in the world
capable of providing working JTAG hardware.
Signed-off-by: Bernd Schmidt <bernds_cb1@t-online.de>
Signed-off-by: Bryan Wu <cooloney@kernel.org>

arch/blackfin/kernel/cplb-mpu/cplbmgr.c

@@ -307,7 +307,7 @@ void flush_switched_cplbs(void)
 	enable_icplb();
 
 	disable_dcplb();
-	for (i = first_mask_dcplb; i < MAX_CPLBS; i++) {
+	for (i = first_switched_dcplb; i < MAX_CPLBS; i++) {
 		dcplb_tbl[i].data = 0;
 		bfin_write32(DCPLB_DATA0 + i * 4, 0);
 	}