ARM: cache-v7: optimise branches in v7_flush_cache_louis

Optimise the branches such that for the majority of unaffected devices,
we avoid needing to execute the errata work-around code path by
branching to start_flush_levels early.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>