[POWERPC] Fix ppc_rtas_progress_show()

Fixes the warning

	arch/powerpc/kernel/rtas-proc.c: In function 'ppc_rtas_progress_show':
	arch/powerpc/kernel/rtas-proc.c:382: warning: the address of
		'progress_led' will always evaluate as 'true'

by fixing the code to do what it presumably is meant to do.

Signed-off-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>