ARM: 9155/1: fix early early_iounmap()

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: b089c31 ("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>

arch/arm/mm/mmu.c

@@ -390,9 +390,9 @@ void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
 	BUILD_BUG_ON(__fix_to_virt(__end_of_fixed_addresses) < FIXADDR_START);
 	BUG_ON(idx >= __end_of_fixed_addresses);
 
-	/* we only support device mappings until pgprot_kernel has been set */
+	/* We support only device mappings before pgprot_kernel is set. */
 	if (WARN_ON(pgprot_val(prot) != pgprot_val(FIXMAP_PAGE_IO) &&
-		    pgprot_val(pgprot_kernel) == 0))
+		    pgprot_val(prot) && pgprot_val(pgprot_kernel) == 0))
 		return;
 
 	if (pgprot_val(prot))