powerpc/mm/radix: Parse disable_radix commandline correctly.

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: 74baf9145fd0 ("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>