[POWERPC] Remove build warnings in windfarm_core

drivers/macintosh/windfarm_core.c: In function 'wf_register_control':
drivers/macintosh/windfarm_core.c:219: warning: ignoring return value of 'device_create_file', declared with attribute warn_unused_result
drivers/macintosh/windfarm_core.c: In function 'wf_register_sensor':
drivers/macintosh/windfarm_core.c:329: warning: ignoring return value of 'device_create_file', declared with attribute warn_unused_result

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>