ARC: Use correct PTAG register for icache flush

This fixes a subtle issue with cache flush which could potentially cause
random userspace crashes because of stale icache lines.

This error crept in when consolidating the cache flush code

Fixes: bd12976c3664 (ARC: cacheflush refactor #3: Unify the {d,i}cache)
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org  # 3.13
Cc: arc-linux-dev@synopsys.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>