powerpc/mm/radix: Use do/while(0) trick for single statement block

In commit 7a22d632 ("powerpc/mm/radix: Update command line parsing for
disable_radix") an `if` statement was added for a possible empty body
(prom_debug).

Fix the following warning, treated as error with W=1:

  arch/powerpc/kernel/prom_init.c:656:46: error: suggest braces around empty body in an ‘if’ statement [-Werror=empty-body]

Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>