powerpc/mm/radix: Parse disable_radix commandline correctly.

kernel parameter disable_radix takes different options
disable_radix=yes|no|1|0 or just disable_radix. When using the later
format we get below error.

 `Malformed early option 'disable_radix'`

Fixes: 1fd6c022 ("powerpc/mm: Add a CONFIG option to choose if radix is used by default")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/init_64.c

@@ -366,7 +366,7 @@ static int __init parse_disable_radix(char *p)
 {
 	bool val;
 
-	if (strlen(p) == 0)
+	if (!p)
 		val = true;
 	else if (kstrtobool(p, &val))
 		return -EINVAL;