powerpc: Fix TLB invalidation on boot on 32-bit

The intent of "flush_tlbs" is to invalidate all TLB entries by doing a
TLB invalidate instruction for all pages in the address range 0 to
0x00400000.  A loop counter is set up at the high value and
decremented by page size.  However, the loop is only done once as the
sense of the conditional branch at the loop end does not match the
setup/decrement.  This fixes it to do the whole range by correcting
the branch condition.

Signed-off-by: Rocky Craig <rocky.craig@hp.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>