Currently __set_fixmap() bails out with a warning when called in early boot
from early_iounmap(). Fix it, and while at it, make the comment a bit easier
to understand.

Cc: <stable@vger.kernel.org>
Fixes: b089c31c ("ARM: 8667/3: Fix memory attribute inconsistencies when using fixmap")
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>