[POWERPC] therm_windtunnel: Eliminate some build warnings

We don't care if the device_create_file calls fail, the driver will work
just as well without them, so just issue a runtime warning.

drivers/macintosh/therm_windtunnel.c: In function 'setup_hardware':
drivers/macintosh/therm_windtunnel.c:268: warning: ignoring return value of 'device_create_file', declared with attribute warn_unused_result
drivers/macintosh/therm_windtunnel.c:269: warning: ignoring return value of 'device_create_file', declared with attribute warn_unused_result

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>