ARM: l2c: restore the behaviour documented above l2c_enable()

l2c_enable() is documented that it must not be called if the cache has
already been enabled.  Unfortunately, commit 6b49241a

 ("ARM: 8259/1:
l2c: Refactor the driver to use commit-like interface") changed this
without updating the comment, for very little reason.  Revert this
change and restore the expected behaviour.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>